The stuff on top? Look like star polyps to me. They appear to be growing on a "base" which has peeled off...which is part of why I would say star polyps. But I may be missine what you are actually talking about.
Check out the hitch hiker thread in the archive. You'll find pictures of both feather dusters and aiptasia. I'm with Ophiura, they do look like star polyps or something related.
i bought a spiny oyster the other day, and from the looks, i have the same thing on him. they look like star polyps, but are brown, but they have the same purple "mat" that they are growing on. hope someone can identify.