They like to pout like that, but keeling over isn't good. Salinity and Ph have to be right on from my experience. If it is a colt coral, I have grown these to basketball size and then lost them in 24 hours when.nothing else was affected. If it starts to liquify, remove the affected arms as fast as possible or they will pollute your tank. It's not that bad yet, but check your water quality and make changes slowly.