I would guess some kind of sea slug. If it is, it'll probably do a great job mowing down any hair algae, if you have any. As far as I know, slugs also are not dangerous if they happen to die. BUT...I'd definitely wait for someone more experienced than me to chime in before dropping it in your tank!! My luck, it's some kind of coral eating Nudibranch...although I don't think so.
Do a google search for Bali Sea Hare. Looks a lot like it. They're also non-toxic but will "ink" when threatened. The "ink" is not toxic though. They do have a habit of being sucked into power heads though.
said this before....I WANT your avatar! Do you have any more pictures of that fish? Although now that I look more closely he/she has a yellow tail. But the expression on that fishes face inspired my screen name .