Squishy Ambush. In the first round of a combat, the slithering jelly has advantage on attack rolls against any single creature it surprised. 

Damage Transfer. While grappling a creature, the slithering jelly takes only half the damage dealt to it, and the creature it is grappling takes the other half. This works both ways.

False Appearance. While the slithering jelly remains motionless, it is indistinguishable from an oily puddle, unless an observer succeeds on a DC 14 Intelligence (Investigation) check.

Liquid Form. The slithering tracker can enter an enemy’s space and stop there. It can also move through a space as narrow as 1 inch wide without squeezing, but may be at disadvantage for 1min, as it regains its form (DM's discretion).

Jelly Climb. If it is above half Hp, the slithering Jelly can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check. If its Hp has gone below half in the last minute, it is at disadvantage on these checks.

Watery Stealth. While underwater, the slithering tracker has advantage on Dexterity (Stealth) checks made to hide. It can use a bonus action to do so.

Jelly's Delight. The slithering Jelly can consume a dead creature. It takes an hour, and may be good for its health. 

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d10 + 3) bludgeoning damage.

Jelly Leech. One Large or smaller creature that the slithering tracker can see within 5 feet of it must succeed on a DC 12 Dexterity saving throw or be grappled (escape DC 13). Until this grapple ends, the target is restrained. If the target doesn't escape by the start of its next turn, it is now unable to breathe unless it can breathe water. 

In addition, the grappled target takes 1d6 necrotic damage at the end of each of its turns if it remains grappled. The slithering tracker can grapple only one target at a time. Once this action is chosen, Jelly will not stop attempting to leech, until the target is dead or unconscious.

Mushy Retreat. As a reaction, Jelly can turn into a thin liquid and both disengage and use it's movement to flee. Jelly is out of initiative and cannot use an action for the next hour. On it's master's turn, Jelly can use its movement, but cannot hinder another creature without affording that creature a reaction attack with advantage. Damage will be considered a critical.