Look up colonized hydroids, I had something's similar, thought they were a type of feather duster ended up behing hydroids, don't freak out at first if that's what they are! My LFS told me I had to tear down and re clean and scrub the entire tank and start over, was not an option! I remover the rock and did I little
Excavation of my own, picked out what I could with tweasers and then chipped rock off to get the roots......so far so good!!!!!!
Hopefully this is not what you have, but it may come in handy for some.
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