Identfy Feathery Slug

[4lti7ude]

Update:
Alright so I fresh water dipped the rock I saw him on. He was on that rock to because I saw him pop out.

Tons of red brittle star fish and black ones and some little worm thing.
But no bristle worm. I let the rock sit in the water a bit and ran some on it too trying to rush him out.

Few days later.
5minutes ago I just saw him coming out the SAME rock. AND HES HUGE!
Hes like 5" LONG NOW.
Hes not eating my frogspawn or my mushrooms. Maybe hes eating the Halimeda Plant in there...

I shot him with PURE concentrated salt water. Now im waiting to see if he survived...