I think they are feather dusters. Im pretty sure they are worms inside of them. They will move around until they find the perfect spot for them (Lighting, flow, etc)
Definitely Aiptasia! get rid of those suckers quick. Easiest way on this system IMO remove the rocks they are on and give them a nice looong freshwate dip or soak them in a 10% bleach solution overnight (rinse for 48 hours or longer in RO water prior to returning them to tank. Good luck
They are not stuck on rock. They were not there yesterday. It looks like they moved from somewhere in the back.
Should I just attempt to pull them out?