Snail or nudibranch?

I just came back from vacation and found two of these no idea what they are can anyone help?

 

Stomatella snails. Great find.

 

So I'm guessing they are good? I found two so far.

 

It's rare to see them during the day. Usually, you'll only see them if you turn on the lights unexpectedly at night. They will reproduce in your tank. If you see "smoke" coming from your rocks at night, turn your protein skimmer off for the night so their whole spawn isn't skimmed away.

 

Yes they are good!
here's some info in it
Stomatella Snails

http://www.aquariumforum.com/f22/stomatella-snails-good-bad-2706.html

 

Ours are out during the day...or they were until the shrimp started eating them. The ones in the fuge are out when the lights are on, though.

 

I wanna build a 48" hang on the back fuge, we'll see......