I had the same thing happen to me this weekend, I originally thought AEFW, but I still have not seen 1, no red bugs, All parms good, dkh 11.5 Ca 450 temp 75-76 SG 1.025 Mg 1350 I did have to raise the Mg last week but it wasn't that low.
So far lost over half my tenius, and some dead spots at the base of a samoensis, and 2 unknown acros.
I did switch salts at the beginning of July when I replaced the tank from IO to Red Sea Coral Pro, thinking of switching back?
Rob
215g sps tank w/90g sump, Emperor Angel, 5 Tangs (Sailfin, 2-yellow, purple & powder brown), pair of GSM clowns, pink spotted and yellow watchman gobies, mandarin, Dwarf lionfish, pair of yellow tail damsels, red coris wrasse, Harlequins