It could be just having an off day. But in the chance that is not the case, how old are your lights , what are their sizes watts k
rating , (sometimes a coral Will crash for no reason by living off their reserves) What are your perameters numbers please . Check spg . Was there anything that may have been sprayed in or around the tank ? Just for added measure you may wish to add some carbon and do a water change ...what are the tanks inhabitints ? That may offer up some clues if something is picking on it or irritating it , any other corals close by that could be irritating it ?