Have you tested your Iodine by chance? I have seen low iodine mentioned in relation to tip burn in multiple threads and that increasing it to 0.06 can protect SPS from tip burn in high light and/or Alk.
I had 1 tip on an SPS frag going white with the stringy filament as well, though in the last day or 2 it seems to be colouring back up. Because it was the only white tip on the frag, I was pretty sure it was not just growth.
Although many experts discount the importance of iodine for corals, in my tank I have found that it directly impacts my SPS and LPS because of a mushroom plague that I have going on. For me it is a bit of catch 22. When my iodine is depleted my SPS frags are unhappy, but the mushrooms are also held in check. When I raise it, things improve for my frags, but the mushrooms start multiplying. :facepalm:
So if you have not looked at iodine, I would consider testing it, especially if you have any amount of soft corals in the tank that could be depleting it and/or you are seeing white tips that lead to recession.
Dennis