The algae on the rock might be irritating it enough that STN is occurring. I would keep an eye on it and make sure parameters are optimal for growth, so it can heal.
Thanks. Looks great except for the underside. Alk is 9, CA 440. Tank is 1.5 year old mixed reef. I’ll check salinity and some other stuff. Piece is halfway up in tank under Steve’s Leds. Running blues at 45% and whites at 20%. Increasing blue 1% per day until I get up in the 60s. Leaving whites as is
It looks mad. I don’t think your issue is just from light. In my experience, Psamacora is super hardy and doesn’t need much light SPS/wise. The part that is alive doesn’t look as fuzzy as it should (see below).
What are your lights, what are your NO3 and PO4? You seem to have a decent amount algae.