What coral is it? Looks like a candy cane coral? Really sorry, but that does not look very good.
We start with what your water parameters are. For me, these are the numbers I need to know for starters. If they check out OK, then we move on to others. But even more important than these values is any significant FLUCTUATION in these values over the last few months preceding the distress you picture.
I will include my "no worry" ranges for these parameters:
Age of tank (+4 months for LPS)
Temperature (around 78F)
Salinity (1.023-1.026 but be consistent)
dKH (7-10 consistent, and dependent on nitrate & phosphate)
Nitrates (5-15)
Phosphates (.06-.16)
Significant volatility in any of these (or zero for nitrates & phosphates) pretty much gives you an answer as to what stressed that coral. So lets just figure this out for your next coral if too late for this one.
ps. None of us are really all that good at "saving" super stressed corals. Just put them around the bottom and out of sight in hopes they figure it out. If algae grows on them, then cut off affected portion.