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I have never seen anything like this sorry I can't be no help so many mysterious things in the saltwater hobby I think that is what makes it so interesting
It almost looks like the residue left behind by mushrooms. They can be cut off or they just let go and leave behind a little mound which eventually grows into new mushrooms. Keep an eye on it and see, last time one of my mushrooms let go I saw they start of a new one growing a week or two later.