If there are signs of brown junk in the area, siphon it off very carefully using a small diameter airline hose. Keep it siphoned off every time you see it so it does not spread to the other heads. Be sure the coral is placed in a med flow area as well so it does not promote accumulation. Nothing strong enough to blow the polyps over but somewhat brisk.
Bring your alkalinity up some, closer to the 150-160 ppm
area and allow the Ca
to drop some closer to 410-415 ppm
. Both will reduce the corals stress some. Check your LFS
to see what vitamin additives they sell. Vita chem being the best for this type of situation. Add double the recommeneded dose directly to the tank every second day and be sure you remove any sorbant media/carbon for a short time.
You need to get those nitrates down as much as possible. Once bacteria sets in, it's "fuel for the fire" so to speak. Several large water changes over the next few days using well aged/aerated SW
will help immensely. Check the chemitry before and after to be sure you stay on target.
How high up the the coral and do you have eggcrate on the tank or any other light difuser?