"Too much light" bleaching would have happened almost initially. If given the light slowly, they can take all of the light that we can throw at them with good spectrum. That being said, I have never once light-acclimated a coral and never had an issue. IMO, this is more of a wrong - spectrum issue.
If they lose color over time, then it is likely high nutrients, lack of major or trace elements or the wrong kind of light.
Losing color underneath is typically a lack of spread-out light source - very common with LED or small-reflector haldies that are heavily prone to shadowing.
It looks to me like yours is starting to STN (slow tissue necrosis). This is not good and hard to reverse in the same tank. Gonna need some parameters to help: salinity, nitrate, phosphate and alk would be a good start.
