Green Blob with a twist!

[Putnam419]

Brand new 29 gallon tank, been cycling for 3 weeks, picked up 3 pounds of live rock from LFS. First time with a salt water tank so my wife and I have been excited discovering all the little hitchhikers.
The Green blob below has us stumped however. We assume it's some kind of algae. But when you try to move it whatever it's attached to visibly moves and clings tighter to the rock! What could the underlying creature be?

http://i61.*******.com/2ueh3i9.jpg

[cloak]

The algae looks like Codium. As far as the underlying creature goes, the algae might be growing on the shell of a clam or a mussle When you pull on the algae, the shell tends to clinch together.

[Putnam419]

The algae looks like Codium. As far as the underlying creature goes, the algae might be growing on the shell of a clam or a mussle When you pull on the algae, the shell tends to clinch together.

Ah, that's what I suspected. I got brave tonight and decided to give it a go and see what's under the Codium, scraping a little at a time I finally uncovered part of it's shell. Definitely some sort of bivalve.