Notyelo, did you ever get an answer to your question? I had the same thing happen to me. My green star polyps were doing great... expanding all over my live rock to the point where I was worried that it was taking over and I'd have to remove some of it. Then suddenly it started dying, flaking off like you said. Nothing else in my tank - corals, fish, inverts, became sick. I couldn't tell if it was some sort of predation or not. My hermits were definitely picking at it, but I'm not convinced they killed it, but rather were being opportunistic when the coral was weakened by something else. My water quality was perfect (confirmed by the local store) although my alkalinity was low, but not low enough to cause dye-off. They couldn't explain it either, although they thought maybe it was a bacteria specific to that species of coral.
That was months ago, and ironically enough a few of the individual polyps hung on and it appears to be making a comeback!
To anyone else who may read this, are there any worms or other nocturnal hitchhikers that may choose to feed on this type of coral, but stays hidden so I'm not aware I have it? I have seen a few strange fuzzy worms late at night, but not long enough to make a good ID.