soft shell snail???

I was finally able to siphon this off my live rock after seeing him a couple times in the past week... it looks like a snail minus the shell and only comes out at night. it's being kept in a plastic breeder tank until i can find out if he's harmful to anything in the tank. thanks for any help you guys can give on the little creature.

 

It might either be a slug, or a stomatella snail. I'm guessing stomatella, as it looks to have the tiny half-shell on it. It'll be in the front half of it, if it has the shell thing.

 

Thumbnail snail (Stomatella varia). Very nice hitchhiker to have.

 

1+ Stomatella

 

Thanks guys! I did a little research on the stomatella snails and you guys were dead on! Thanks for the quick replies and I'll be returning him to where I caught him this evening.