Did you test your water parameters? We would need that to try to come up with something... It seems that something really upset that coral (seems to be an euphylia?)
All water peramiters were good. All other corals in my tank are doing healthy. Salinity 1.026. Ph 8.1+/-. Nitrite/ nitrate reading 0. Ammonia still showing 0 from test and stick on reading. Calcium 440. DkH 8.3. Temp is 24c. I don’t run a sump setting on my tank it is just a basic fish tank filtration system. I have a protein skimmer. Two power heads too help with the coral care of water cycling I also have a pretty good clean up crew that are healthy too. Two days prior I did give my corals a feeding of reef roids. As usual the feeding types closed up and re opened later on. I just find it weird I wake up and find this. Since I have gotten back from work it has started too look worse. I am in the process of doing a 15 gallon water change right now. Just mixing up the water.