What is this on my live rock?

[Gladmaker]

A week ago I got some new live rock that came from the Gulf. It had something that looked like a sponge that was dying. When I took it out of the bag, it felt like it had a shell. After it was in the tank for a day, it had a lot of soft stuff around it. I removed the mess and left what felt firm. My blenny and crabs have cleaned up the rest of the soft stuff. Today a hermit crab was crawling over it and it appeared to be loose and blowing in the current. I reached in and knocked the crab off and attempted to remove the object thinking it would easily come off. It was now stuck tight to the rock and has a firm shell. I didn't try to force it since it must be alive to be able to quickly stick to the rock.

 

[Gladmaker]

I had another say it may be a turkey wing clam, but the shell doesn't feel as hard as a clam shell. In fact after I put it in the tank, it was soft. I didn't feel of it again until today and now it's firm, not hard.

 

[AlSimmons]

Fleshy Limpet perhaps?

 

[pagojoe]

It's an Ark Clam. It feels softish because of the stuff on the outside of the shell.