You might want to inspect the rock where the coral is attach........look for asterina starfish. I inspect my zoas or palys when they are closed or they are diminishing, and the only thing I have found munching on them are this small starfishes. What I do is, use a magnifying glass to check thoroughly......and usually find them and their little offsprings (arms)....that's how they reproduce. Also, after the inspection, I dip them in mixture of iodine and tank water....let them sit in solution for about 10 to 15 minutes and inspect again. This also helps if any other pests are lurking around.