Traveling atm, so cant post pics, but I assure you it is Chaeto. As small pieces brake loose, they will catch on rock and then grow. Perhaps 'attached' to the rock is a bad term, but I assure that it at least creeps along it and grows in the same clump fashion that is familiar. I agree that Chaeto is normally a free floating species in sumps ( that is where I have a mass of it), but in this case it is loose in the tank.
It is easy to pull out, and not really a bother to anything, other than it is just something else to have to deal with. THere has to be something out there that will eat it.....
Any thoughts are appreciated.