Clam?

[IlliniMed]

So I did a little surveying and I noticed this thing popped up right in the middle of my fruit loops. I poked it and it seemed to close up and then open again a few seconds later? Any ideas ... ?

 

[Crabs McJones]

Looks like a clam or a bivalve @DSC reef

 

[Oshengems]

Looks like a baby derasa clam

 

[DSC reef]

Definitely a bivalve, maybe a type of oyster. Doesn't look to be a tridacna though.

 

[sfin52]

I find these on a lot of zoa frags or colones at my lfs. One frag I bought had three. One died the others two are doing well. Nice find.