Large sps colony dying from the bottom?

[Aquattro]

I'd look for pests or something stinging it. Sure, large colonies can start to fade at the bottom, but not in a day or two from a healthy colony. It's location is a premium spot in the tank, lots of flow and light. I do 50% water changes all the time, so I doubt that either, and it wouldn't limit the effect to one colony. something else going on here.