What is this? PICS!!

[revhtree]

I have 2 oyster like looking things attached to a zoo colony. The have like little tenticles sticking out from there shell and they dont move it seems. Anybody know about these things?

TIA!

 

[revhtree]

Here are some pics:

I am wondering if this is the reason these are the only poylps doing bad in the tank. Look at how they are not spread open.

 

[Fluff]

It almost looks like a barnacle but, I'm gonna guess it's a clam of some sort. :?