2 months ago, a couple colonies of mine did the same thing. I lost 70% each of a hydnophora, pocillipora and a neon green acro. Untouched/unaffected were my birdsnest, and a purple acro.
I never could find anything wrong, and chaulked it up to the fact that right now I have a "coral garden" set-up (mixed soft, lps, sps) and too much chemical wafare going on.
Then it just stopped. I still don't know why. All the while the corals were dying off, there was full polyp extension and good color on the remaining living tissue. Never lost an entire colony. They're all growing well again too.
<shrug>
Mitch
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