I know it's been a while, but I wanted to contribute my one data point: My cali tort frag from the same mother colony started doing almost the same thing this week - slow tissue loss from the base, on the shaded side facing current. No tissue loss on any neighboring corals. No detectable red bugs, but I've had alk drop to 6 dKH (usually run ~7.5). Polyps are otherwise totally hanging out and happy, and the whole thing has been growing and coloring up since I got it from you in November.