I couldn't find any search results in the forms for 'half-merfolk' or 'mertouched', so I'm posting this here to ask for advice about bringing a 3.5e race to 5e.

This was the only wiki info I could find on a half-merfolk/half-siren race

= Type: Humanoid, Aquatic
= +2 Dexterity, +3 Charisma. Half-Merfolk are lithe, agile, and graceful and inherit the playfulness, charm, and beauty of Merfolk.
= Medium Size. As medium creatures, half-merfolk receive no bonus on attacks or AC.
= Speed: 30 ft.
= Swim Speed 30 ft. Half-Merfolk gain a +8 racial bonus on all Swim checks and can always choose to take 10 on Swim checks, even if rushed or threatened. Half-Merfolk can use the run action while swimming, as long as they swim in a straight line.
= Amphibious. Half-Merfolk can breathe both water and air.
= Low-light vision. Half-Merfolk can see twice as far as Humans can in starlight, moonlight, torchlight, and similar conditions of poor illumination. They retain the ability to distinguish color and detail in these conditions.
= Weapon Proficiencies: Light crossbow, heavy crossbow, trident, and net. Merfolk use these weapons famously, and Half-Merfolk train with them as well.
= +4 racial bonus on Bluff and Gather Information checks when interacting with a Humanoid.
= Automatic Languages: Common, Aquan, and home region. Bonus Languages: Any (excluding secret languages such as Druidic).
= Favored class: Bard
= Level Adjustment: 0

Here's all the things I can tell for starters.
-Swimming is not listed as a skill on 5e sheets.
-Bluff is Deception. Gather Information is probably Investigation.
-A total of 5 stat points is slightly higher than average for racial bonus. Most 5e classes in the standard handbook get 3 total stat points, some get 4, and humans get 6 total since they get one point in each.
-4 languages is more than any race starts with. Aquan isn't really one of the most used languages, but that still gives 1 language based on where they were raised, and one free non-secret language.
-The only mention of Low-light vision in the entire 5e player's handbook index was "see darkvision" and darkvision mentioned low-light as a condition, but not as a racial ability, which makes me wonder if darkvision has replaced it.

Any help and ideas would be fantastic. I'm excited to have this race for 5e campaigns.

Check out the Triton race from Volo's Guide to Monsters for something similar enough to use, or at least to start from.

Indeed the Triton race is the playable "merfolk" of 5E. The race is introduced in Volo's Guide to Monsters as a playable character race. They are described thusly: "Long-established guardians of the deep ocean floor, in recent years the noble tritons have become increasingly active in the world above."

A lot of the information you found seems, to me, significantly unbalanced. Hope that helps.

Elemental Evil Player's Companion introduced playable Water Genasi, which have similar traits to tritons.

Mordenkainen's Tome of Foes will introduce playable aquatic elves.

Sword Coast Adventurer's Guide has a variant rule for half-elves that lets them trade Skill Versatility for a racial trait from their elf heritage. Half-elves with with aquatic elf heritage can gain 30 feet swimming speed, though they don't gain the ability to breathe water.

You could also use the merfolk races in Plane Shift Zendikar and Ixalan. 

I think someone did put a homebrew Merfolk race on the d&d wiki, with art from the MTG merfolk cards. Merfolk Erectus (5e Race)

but Tritons would be the closest thing to Sirens and Merfolk in d&d, I don't imagine sirens would have control over the water like Water Genasi would. It wouldn't be d&d without a questionable amount of half n' half races.

Water Genasi can move/create/destroy water, gain acid resist, and gain bonuses to Con and Wis. They have swim speed and can breath underwater. They live to be up to 120.
Tritons get Fog Cloud, Gust of Wind, Wall of Water, and cold resist. They get a bonus to Str, Con, and Cha. They have swim speed and can breath underwater. They live to be up to 200 and can talk to seacreatures.

Taking that into account, Half-merfolk looked like a Dex/Cha-based race, with no special magic or resistances.
= 5 points is too much, the Cha bonus would have to be dropped from 3 to 1, to highlight their dexterity.
= 4 languages with one of them being free is probably too much. It looks like Primordial is the replacement for aquan, and if they're getting 1 free language based on where they were born/raised, the '1 free non-secret' language seems too much as well.
= Lowlight vision will probably be bumped up to Darkvision
= Actually, instead of Darkvision forever, we can take that bit from the homebrew merfolk page and only give it to them while underwater, this seemed helpful for them to have to make up for it and make them more unique.
Underwater Capability. Underwater combat does not impose any penalties or disadvantages on movement or weapon attacks. While underwater you have darkvision, you can see in dim light within 120 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
Slippery. You have advantage on dexterity saving throws against being grappled or restrained.
= That complicated bonus to Swim checks and taking the 10 will probably need to be removed or lowered if there's nothing appropriate for Swim to factor into.
=Cold or Acid resist seems like it could work, but probably at the cost of that +4 to humanoid investigation and deception.
=I don't know a lot of lore about underwater/amphibious races, but I don't think a lifespan of almost-1000 would sit right with merfolk. 300 or 400 at the very most I would think, for a pureblood merfolk. Which would leave a half-merfolk around 200-275.

= Type: Humanoid, Aquatic
= +2 Dexterity, +1 Charisma. Half-Merfolk are lithe, agile, and graceful and inherit the playfulness, charm, and beauty of Merfolk.
= Medium Size. As medium creatures, half-merfolk receive no bonus on attacks or AC.
= Speed: 30 ft.
= Swim Speed 30 ft. Half-Merfolk can use the run action while swimming, as long as they swim in a straight line.
= Amphibious. Half-Merfolk can breathe both water and air.
= Underwater Capability. Underwater combat does not impose any penalties or disadvantages on movement or weapon attacks. While underwater you have darkvision, you can see in dim light within 120 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
= Slippery. You have advantage on dexterity saving throws against being grappled or restrained.
= Weapon Proficiencies: Light crossbow, heavy crossbow, trident, and net. Merfolk use these weapons famously, and Half-Merfolk train with them as well.
= Resistance to Acid damage
= Automatic Languages: Common, Aquan, and home region.
=Age: Mertouched are rare, and there aren't many records to pin down their age. The best guess would put them somewhere just beyond a 200 year lifespan, but well below 300.

I think it looks a little better at least now.

Quote from Pirostyle >>

 = Type: Humanoid, Aquatic
= +2 Dexterity, +1 Charisma. Half-Merfolk are lithe, agile, and graceful and inherit the playfulness, charm, and beauty of Merfolk.
= Medium Size. As medium creatures, half-merfolk receive no bonus on attacks or AC.
= Speed: 30 ft.
= Swim Speed 30 ft. Half-Merfolk can use the run action while swimming, as long as they swim in a straight line.
= Amphibious. Half-Merfolk can breathe both water and air.
= Underwater Capability. Underwater combat does not impose any penalties or disadvantages on movement or weapon attacks. While underwater you have darkvision, you can see in dim light within 120 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
= Slippery. You have advantage on dexterity saving throws against being grappled or restrained.
= Weapon Proficiencies: Light crossbow, heavy crossbow, trident, and net. Merfolk use these weapons famously, and Half-Merfolk train with them as well.
= Resistance to Acid damage
= Automatic Languages: Common, Aquan, and home region.
=Age: Mertouched are rare, and there aren't many records to pin down their age. The best guess would put them somewhere just beyond a 200 year lifespan, but well below 300.

I think it looks a little better at least now.

• There's a lot here; this doesn't seem like a half-race at all. Slippery and acid resistance aside, this is basically a Sea Elf.
• Acid resistance and slippery don't seem to have any justification. The only reason water genasi have acid resistance is that they're part water elemental. None of the published aquatic races have anything like the Slippery trait.
• "can use the run action while swimming, as long as they swim in a straight line" is redundant (and the straight line restriction is silly). Anyone can use the Dash action with any of their speeds.
• Half of Underwater Capability is redundant with what a swimming speed does. The way it's worded also takes away disadvantage to underwater ranged weapon attacks, which makes no sense.

I think that sea elves and half-sea elves do exactly what you want (elves are already described as having otherworldly grace).

Quote from InquisitiveCoder >>

• There's a lot here; this doesn't seem like a half-race at all. Slippery and acid resistance aside, this is basically a Sea Elf.
• Acid resistance and slippery don't seem to have any justification. The only reason water genasi have acid resistance is that they're part water elemental. None of the published aquatic races have anything like the Slippery trait.
• "can use the run action while swimming, as long as they swim in a straight line" is redundant (and the straight line restriction is silly). Anyone can use the Dash action with any of their speeds.
• Half of Underwater Capability is redundant with what a swimming speed does. The way it's worded also takes away disadvantage to underwater ranged weapon attacks, which makes no sense.

I think that sea elves and half-sea elves do exactly what you want (elves are already described as having otherworldly grace).