Clam or muscle?

[cmn3]

Ive noticed that on one of my live rocks has what looks like a clam on it it is same color as rock and I thought it was just a shell but when my emerald crab got to close it closed up then later it opened back up should I just leave it? Its about the size and shape of a steamer clam and is covered in purple coralline..... Just keep finding cool stuff in my new setup

 

[akunochi]

Probably a muscle. I had a couple in my last tank....if you can post a pic it will help

 

[user 26995]

Yeah a pic would be cool. And its definately a keeper.

 

[cmn3]

I will post a pic tomorrow at work now lights will be off by the time I make it back