you may have crabs in the tank they will eat polyps I caught one and some SPS and secured it. turn lights off and about 1 hour latter go check out your tank. or put a piece of sprimp in a glass and crab will try and get at it the shrimp will also so put it close to the rock if shrimp are in it put net over and place shrimp in pump. if crab is in the cup then remove and place in toilet and flush.

Bill