Is this some type of sponge, or algae?

[ggoodrich]

This red and white fluffy looking thing is actually growing on some sort of a mollusk (I'm guessing). Just wondering for certain what it is. It hitchhiked in on some aquacultured rock from Gulf-View. Also, any ideas on what this "mollusk" is? I purchased 60lbs of live rock from Gulf-View, and there are quite a few of these, probably at least one on every piece. Their hinge is facing out, the opening of their shell facing the rock. They do seem to move a bit, but they don't change locations, just tilt a bit, or somehow move out away from the rock a little. You can see their shells open up to 1/8" or so, but if startled, they quickly close and tighten up against the rock. Most of them are about 2" long and have kind of a "hip" bump near the hinge facing out.

 

[dolfinatic]

The red is a sponge. The white fluffy thing I believe is a type of bryozoan. Cant tell what the mollusc is, it is covered up too much.