I'd do a test for DOM (dissolved organic material) and you might consider water flow issues if the bleaching is limited to one area of your tank. I don't think the ph of 7.9 is anything to worry about, unless it's accompanied by low oxygen levels or high organics (those tend to go hand in hand)

78F is a little low for most acros; I'd bump it up to 80F and keep it there. I'd also make sure your salinity/specific gravity is accurate, meaning use a refractometer or lab grade hydrometer. You can also check for phosphate, but if you have high phosphate levels you probably also have a lot of algae. How about calcium level, what's that at? How are you repenishing calcium and carbonate?

Assuming that calcium, S.G. and water movement are all in good shape, I'd bet this might have something to do with high organics in the water. It's the sort of thing that can creep up on you over a period of time, then stony corals will start to bleach. Once it starts, you might have trouble containing it. Good luck!!