New tank live rock going grey

[Aquattro]

Hi, welcome to the board.

First, 10 yo rock is potentially full of nutrients. When you say water is tested, please list parameters, it helps a lot. NO3, PO4, NH3, salinity, temp.
Also, give details on flow and lighting.

It sounds like you might need a bit more flow over that area, maybe get some oxygenated water flowing over it. It could also be a dying piece of sponge, they often go gray.
Without a pic, we're just guessing.