Help ID a LR HH?

[victoria]

Last night I was staring at my tank and swore a rock moved. Watched for a while, and sure enough - there's a section of *really* live rock in there. My best guess is a kind of bivalve, since it seems to open and close like one, and reacts to light. I (gently) tried to move it with a butter knife and it wouldn't budge.

It's that whole section with the polyps on it that moves.

Any ideas?

Thanks!
Victoria