I've removed every nudibranch that came in with my live rock. Too much of a risk it'll eat my corals. I donated them to a local reefer trying to raise them, and turns out they do eat aiptasia!
Looks like an elysia to me which would be reef safe but will almost certainly perish without a lot of a specific kind of macroalgae (depends on the species) I am really not sure on the ID and outside of those and a few ugly algae eaters I believe almost every nudi in the sea is either not reef safe or completely unsuitable for hobbyist aquariums due to a specific diet. I would not ever keep one if I was not 100% sure what it was and that I could keep it.