Sorry to see that.It could be reacting to a change of light intensity.Either too much or too little.Your Nitrates seem a tad low for LPS. Maybe add some more frozen foods to your fishes diet.If you know someone with a coral saw ,I would recommend getting this trimmed.Sometimes the local Aquarium store is willing to do this for you. Getting a trim will help with the regrowth process in these types of corals.You may want to keep a eye on your pH,make sure it doesn't go lower.And last but not least check your Salinity!