First off, Welcome to Reef Central!!!!!!!
Depending on the size of the colony... Most corals die off a bit at the base when they are completely shaded of light... Iif that's not the case, maybe there was a quick shift in either ALK or Phosphate recently , which can also cause a slow recession of tissue called Slow Tissue Necrosis or (STN) for short.. Give it some tame and some stable parameters and it should recover. Make sure that you don't have too much direct flow pointed at the coral as well..
Good luck!