Numbers look good. The only thing that I’ve seen that looks in any way similar to that ashy stuff on the mat of the paly, is detritus. Could also be some pests waste. Urchins leave behind similar waste when they poop. Other questions that come to mind is if there is a particular fish that might be picking at them? Hows your water flow? Any new additions to the tank that might have brought in a hitchhiker? Anything has been trimmed or cut off that might have released a substance into the water column? Any changes in your routines? Like adding any new additives or reactors, maybe RO could be contaminated or over spray from cleaning chemicals. Just brainstorming here. The middle of the night when the lights are off is a good time to try to spot any pests feeding on your corals. These are all questions that come to mind when trying to solve that problem. Hopefully it helps & hopefully someone that has experienced this issue can give some input. Good luck.