Noticed a brownish patch with white spots 2 days ago on one of my mushrooms. Next day the patch was darker and slightly bigger. Talked to my local reef shop owner and he said it might be flatworm and to try brushing them off with a soft brush. When i tried that it became apparent that the tissue was dead (it disintegrated when brushed). Now I am wondering if it is slow tissue necrosis and what I can do about it.Water was checked 4 days ago and I was told it was all fine.