there can be a few causes. Directed water flow, placement causing irritation or too close to a nearby coral, water parameters or chem and lighting.
My guess is the ammonia reading is not accurate considering the NO2
unless you've been using antibiotics. The Chem seems fine except the Ca
is a tad high although nothing to get alarmed about. So that narrows it down a bit.
I would think it's actually a placement issues. Either directed luminar flow or another coral. If you can post a pic of the recession and the general area the coral is in that would help. Moving it off the rock where it has attached may be a challenge but it does seem like the best option without more info. These types of corals often start out attached to rocks but as the mature they release from the rock structures to dwell on the sand.