Please ID

So I got a rock covered in polyps, after a few days I noticed this flat thing sticking up out of the polyps. It is black at the base and goes clear as it gets to the top. And it sometimes opens I haven't seen anything come out of it and I cant see down in it, to see if there is anything there. Any ideas what it might be?

 

could you post a picture?

 

Ok so it definitely opens and closes, and it is soft on the top.

 

iTS A clam ( muscle , have one on a frag of zoa's.. it's harmless , also it consumes nitrates , but its so small

I have sevral in my fuge the size of half dollars

 

Thanks that was kinda what I was thinking but I figured I would ask some one.

Is there any thing special I should be doing with it?

 

nope filter feeders, if you got a sump or fuge stick it in there

 

nice find!

 

be carefull its small and delicate and trying to remove it may kill it. leave it be if at all possible.

 

Its not hurting anything, and the tank is a small 22 cube so he will just be staying where he is. Thanks for all the information.