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.
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Brad
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