Immediately I would think too much light intensity and would move it down toward the bottom of the tank if its not there already.
IF on the bottom I would place it in a location away from the strongest of your lighting.
I have had same corals of different colorations where one would do great in high light intensity while the other needed super low intensity to maintain coloration.
You can kill a coral a lot faster with too strong of a light Vs lower light.
It does appear that the coral is shaded a bit on one side and no light coloration is seen while the other half appears to be suffering.