Slug? Cucumber? Pics included

Hes in the middle on the rock between some Cyano. kinda hard to see im sorry!

Just knocked this guy off the glass while cleaning. I thought it was a snail but it curled up while it was floating around the tank. it landed on this rock and has a white underbelly with 2 small antennas on his head. crawls around like a slug maybe a sea cucumber...IDK. anyone have any good ideas?

As always, thanks in advance and I'm still trying to take a better picture of him.

 

If it has a small, woefully inadequate shell, probably a cap snail (stomatella varia). Good thing to have.

 

Can you circle it in the pic or something? I don't see anything.
Servillius has a good guess, though. That would be my guess from description.

 

It is the blue/grey spotted patch in the center of the picture.

Agree if it has a shell it is most likely a Stomatella varia. If not I think it could be a slug or nudibranch.

 

Didnt seem to have a shell when it was floating around. It curled up and didnt look like it had a shell. It did move pretty quickly once it was on the rock. Ill keep my eyes peeled for him and try to get some better pictures..... If only a macro lens wasnt so expensive

 

LOL they are pricey. Stomatella can scoot when they want to. They only have a half shell on the top.