nudibranch?

[Spicemonkey]

I am unsure what this is. I have a inkling it is a nudi tho outer shell slightly hard

 

[mr_X]

That is a harmless and beneficial Stomatella snail.

 

[Dude1982]

Great little guys to have, I have hundreds and are great clean up crew, they reproduce by spitting out a velvety smoke full of eggs