I have a theoretical question concerning Spell Mastery and feats that grant wizard spells like Fey Touched.

I've looked this up on the internet and this forum, but I've not found an answer.

In this example, I'm talking about Misty Step, while you have the feat Fey Touched.

Do you still need to prepare the spell to use it with Spell Mastery?
Or is the spell considered as "always prepared"?

Feats such as fey touched specify that you "learn" the spells they give. Those spells are considered spells known and are always available without being prepared (like how a sorcerer or other known caster always has their known spells available), but they are completely independent of any class-based spellcasting. A wizard with fey touched cannot prepare or unprepare the spells that they get from the feat, unless they scribe them into their spellbook, but even if they do that and don't prepare those spells, they're still always available. (They can prepare them as well in that case, but that would be redundant.)

Spell mastery specifies that you choose a 1st level wizard spell and a 2nd level wizard spell. Fey touched (or similar feat) spells are completely independent of wizard spells as I mentioned, so they only qualify if they are scribed into the spellbook.

It's also worth noting that if a wizard does want to scribe their fey touched spells into their spellbook, there's no RAW way to scribe spells from the wizard spell list that are known through other means (multiclassing, feats, etc.) into the spellbook, so they would have to scribe a spell scroll of the spell first and then scribe it into their spellbook from the scroll.

Thank you for this explanation. So, if they have the spell scribed in their spellbook, later gain access to the same spell through a feat that allows spell slots to be used to cast the spell again, and later use this spell for spell mastery, they don´t need to prepare it again. Because thanks to the feat the spell is always prepared.

Quote from GishLover >>

Thank you for this explanation. So, if they have the spell scribed in their spellbook, later gain access to the same spell through a feat that allows spell slots to be used to cast the spell again, and later use this spell for spell mastery, they don´t need to prepare it again. Because thanks to the feat the spell is always prepared.

 If they have the spell both in their spellbook (which counts as a wizard spell) and from a feat (which doesn't), then they don't need to prepare it as a wizard spell in order to cast it because it's always known through the feat.

However, spell mastery only affects it if it is prepared as a wizard spell, so even if it's known through a feat then they still need to prepare it as a wizard spell. You could still cast it because of the feat, but beyond the one free cast that the feat gives you, it would still expend a spell slot unless it's also prepared.

Spell Mastery lets you pick a 1st and 2nd level wizard spell from your spellbook. The chosen spells can be cast for free, infinitely, at base level without using spell slots while the spells are prepared.

Learning a spell is not the same as having it in your spellbook. 

You would need to scribe the spell. By RAW you can only scribe a spell into your spellbook if it is a wizard spell or on the wizard spell list and you have a visual reference of it (so as was mentioned, RAW you'd need to make a spell scroll of it) OR if you already have the spell prepped and just copying the spell (such as making a backup book). Alternatively, if it is a wizard spell for you, you can select it as part of your 2 free on levelling up and add them that way. You'd only need to prep the spell if you intend to then select it for Spell Mastery.

Personally, I'd let you scribe it into your book as if it was a prepped spell, provided it was a wizard spell. But that is a houserule not the RAW.