Could this be a clam or scallop?

I purchased a rock filled with purple rics on Thursday. The rics are attached to what seems to be a clam, mussle, or scallop shell. Common from what I've read. Well I noticed it moved a bit from it's spot. Yes I found that weird because I figured it wasn't alive as originally it was closed and I thought it a rock.

Now I am not sure if it's alive but it is slightly opened. Now usually when you go near or slightly touch my flame scallop it closes up real quick. So of course I tried tapping it lightly and nothing happens yet if you pick it up it closes and it feels heavy not empty.

Is there another way of detecting if it is alive? What are the chances?
Thought I'd post this bizarre occurance and send some pics.

 

If it's moving it is probably alive. You should be able to inside of it when it is open. Here's mine:

http://www.3reef.com/forums/general-reef-topics/scallop-hitchiker-37390.html?highlight=scallop