Oh, that ain't good, sounds like he's on his way out. Sorry to hear that.

So sorry to hear about this. I once lost five fish and three shrimp over a twelve hour period for no apparent reason so I know where you are coming from. It sounds like it is too far gone to save it now. If you are looking at the skeleton I would guess that is all that is left of your plate coral.

Leave the skeleton in your tank for a little while - watch how quickly it turns green... AFAIK, if it turns green quickly, you may have a phosphate problem... Experts?