What is this? Some sort of clam?

[Shootermcg]

I bought a colony of zoas a couple of months back. About a week ago when I was doing my weekly water change I must have knocked it because I realised I could see underneath the rock. I seen something there but I thought it was a pineapple sponge but looking at it just there I'm sure it was opening and closing like a shell. So I snapped some pictures and thought I'd put it to us to see what us think. It's hard to get a descent pic because of where it is. Thanks

 

[KJ]

Bivalve (Clam, Oyster, Mussel)

 

[VKP01]

Bivalve (Clam, Oyster, Mussel)

Seconded. They end up on rocks sometimes, harmless and fun to watch open and close as you work around them!