This coral is dying, there is tissue recession all over it. The algae/dinos are taking over the newly exposed skeleton as there’s no established microbiome on it yet. My guess is you don’t have stability in the tank for Stoney corals (ie: consistent testing, and water changes and/or dosing). Stoney corals love rock solid consistency and great water quality. Large water changes are not good either as they may cause drastic changes in parameters if the salt isn’t equally matched to where you keep parameters.Heres a better pic of the lepto
Seeing all the algae in some of your previous photos I will start with a focus on nutrient levels in the tank with good test kits like salifert or hanna. My guess is your nutrients aren’t where you think they are. If your nitrates are at 1ppm then you may very well be seeing dinoflagellates.
You’re also running low alk and cal. What is your maintenance routine for the tank and how do you keep the parameters you aim for?