I have something like that. Mine seem to be "pineapple sponge". They have feather like things at the top of them. They just arrived in huge numbers. My tank is older with no new live rock for 2 years! How old is your tank? They cover one side of my sump.
My tank is almost 2 years now with no additional live rock. I think this is pineapple sponge. They just come out a few days ago. I also ask some of my friends that this is harmless.
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