That means they are not happy. First thing to do is check for pests. Flatworms, parasitic copepods/isopods, vermatid snails etc. The best way to find flatworms is with a flashlight a couple hours after the lights go off. Look for bite marks on the underside of the corals if it’s flatworms. Also look for any bugs crawling on the flesh. Any bug that is found on acropora flesh is a bad bug. Good bugs like amphipods and some copepods do not venture on to coral flesh.