These are good points, but it would also help to know if the coral was being fed directly. If so, I doubt the low NO3 was a problem. We kept these for like two decades in low nutrient tanks and they thrived when fed (even things like mysis, lol).
I agree that if it's in high light, the coral should get less light, especially given it's compromised state.
Either way, there looks to be a bacterial issue now. Probably worth trying to treat it for the bacteria and in the meantime consider the possiblity of raising nutrients. (Honestly if the rest of the tank is thriving I would be careful with changes.)
OP, my experience is that these do NOT like a lot of flow. (Edit: that is based on sps standards, not jaw breaker mushroom standards. I guess that means medium flow)
Second edit: 260 par is quite high for this coral. You're probably sizzling it. Does it look crispy?