looks like red cotton or red turf algae. i've had it before and it sucks because its not the easiest to rip off the rock unlike cyano which can easily be blown off. i got a couple turbo snails that got rid of it. i have heard that some rabbitfish pick at it as well but i've never tried that. in the end, the turbo snails did the trick for me, but i did have to take one rock out and hand scrub it with a knife, toothbrush and tweezers before the turbos could do their work. hope that helps.
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