Help, M. Digitata bleach/die off

[mark]

Over about the past two weeks on most of both of my green and purple (though always a little brown) colonies of M. Digitata the polyps have fully retracted and the tissue between have faded to white. The effect is sort of patchy, some problems started on the top, other pieces on the side or bottom.

Nothing added, nothing changed, other than a slight drop in my pH to 7.9 midday, from 8.1, Ca 380, Alk 10.2, temp ~79 with no spikes or drops. NO3 would say 0.0 to 0.2.

My acros have picked up colour over the last month (got a little bleached and browned out after my little red bug problem last fall). The plating Montipora is unchanged in colour and growing. Same with the Xenia, GSP, Frogspawn, Brain, Tubipora, zoos and button polyps, all okay. Fish and clam also seem without problems.

Any ideas, other that pH which I'm going to start buffering with Seachem Reef Buffer.