You might keep a look for redbugs. It looks like what was happening to my sps when I noticed redbugs. They come out at night and destroyed very large colonies almost over night. Also what is got other tank parameters
I looked for red bug and aefw. I didn't see anything. My tank is loaded with sps and I do not see any damage on any other coral. I trimmed the top half of the coral that was not damaged and the pic is of the base that was left.
If it was red bug I think the coral would have died long ago.
Could it have been that there was not enough flow to keep detritus from collecting there, and the detritus was blocking that spot from flow, and light?
Pic of the top half of the coral. I cut the top half off that had no damage dipped the frag and re mounted it to a new plug.
Now after about 24 hour the frag is showing signs of tissue loss
