Hard to say without details about your tank, water parameters, flow, lighting, etc. I have 3 montis that are thriving with around 135-150 PAR, so they don't necessarily need extreme amounts of light.
Similar issue. This coral was doing well and now all of the sudden it looks like it’s losing tissue. Would low flow do this? Lighting? My nitrate and phosphate may be a little high and all hasn’t been stable. It went down to 6.5dkh but I got it back to 9dkh over the last week. I also am struggling with dino algae - could that be the culprit? The weird thing is, a couple of months ago I lost a frogspawn that was in the exact same location - coincidence or is there something about that location on the rock?