Hole in my Red Bali Star

[jimmichal]

Yesterday morning my Red Bali Star (Frormia milleporella) looked fine but this morning most of the central disc is gone. There is just a thin ring of flesh holding the legs together. It's still alive because it is moving across the glass. Anybody have any ideas on what might have happened? I've had it since June and I keep it in a 30 gallon breeder with a rusty angel, a yellow watchman goby, a perculla clown, six line wrasse and a tail spot blenny. I also have a pisto; shrimp and several turbo snails.