Not tube worms nor fd's. If it wasn't on the little rock with my wee bitty birds nest I'd let it grow just to see what it does. I think it may be plant related as it grows back within a couple of weeks after removal. No spreading yet pray pray pray...I'll try an get a closer shot of it. Maybe somebody knows. Hey! I'll ask Mikey-he knows everything. LMAO.
Is it possible the second pic is more mini brittles? They like to get themselves into rock, and wave their arms into the water colomn to collect food. A possibility perhaps?
You know the saying, where there is 1 there is 100's! True saying too, regarding mini brittles most of the time!