Do I see biopsis? That film can be dino or cyano. Cyano super easy to fix, if dino then having this tank size I would restart the system. Microscope can confirm.
What I would do is just to remove that rock, scrub it and rinse fresh salt water. Clean everything, add filter socks to catch what you scrubbed. And repeat if issues are back. If you are afraid of corals just remove them for this process and keep on a rack or something.
In the meantime stabilize parameters and keep your phosphate/nitrate low but not 0.
Unfortunately, this type of neglect is kind of severe (but common, don't blame yourself too much. it is part of the hobby) and you will have many, many different advises. Different schools, different approaches but for sure you have to figure out what happened exactly so it will not come back.
What I would do is just to remove that rock, scrub it and rinse fresh salt water. Clean everything, add filter socks to catch what you scrubbed. And repeat if issues are back. If you are afraid of corals just remove them for this process and keep on a rack or something.
In the meantime stabilize parameters and keep your phosphate/nitrate low but not 0.
Unfortunately, this type of neglect is kind of severe (but common, don't blame yourself too much. it is part of the hobby) and you will have many, many different advises. Different schools, different approaches but for sure you have to figure out what happened exactly so it will not come back.