I'm not convinced about high mg being the culprit.
My mg was off the chart for over a month when my SG was too high, did not lose a coral.
It also could have been Mg dosed too quickly. I'm suspicious of the Mg, since the Mg dosing was the only thing done differently while I was gone. My other RSM tank, that still has the stock bulbs, also had some bleaching, and the Mg was really high in that tank too. But, the important thing to me now is that everything seems to be stable!
To be safe, I'm going to automate my Mg dosing from now on too. Just ordered some more BRS dosing pumps.