A few things to identify

[Belladauna]

Pretty sure he's a sea hare. I saw giant ones at the store here in Calgary today, showed one of the aquarium techs a vid and that's what they seemed to think. Only difference was the ones In The store were yellow. They both crapped out of the same spot on Their backs though, same body and distinct heads.

 

[Belladauna]

http://www.seaslugforum.net/doladola.htm


Found this. This appears to be him.

 

[mr_X]

Oh, I was looking at something completely different. Yes, it does appear to be a sea hare. Awesome hitch hiker. If you don't want it, give it to someone with an algae problem.
It won't hurt your tank if it's small.