So many factors to consider when figuring out why it's bleaching. Couple things though, how rapidly is it bleaching? Are there other SPS (acros in particular) in the tank, if so how are they doing? When I see SPS bleaching I usually ask how long has it been in that spot (just so you can either rule out light or not), other than that I'd check things like KH, KH is important but more important than a particular number is, is that number stable. Things like temp swings, SG swings can also wipe out SPS. And honestly, sometimes SPS just die, I just recently had an acro go from very dull brown(previous owner didn't have nowhere near enough light) to coloring up nicely to completely dead in about 3 weeks, meanwhile the other pieces in the tank were doing fine.