Yeah looks like can see the feather part off the feather duster/tube worm, when zoas are closed do you see a white calcerous tube?
If so tube worm and worm lives in tube and sends out that fan to catch food to eat
No not harmful, but occasionally see others post,can get to plague proportions on overflows and pipework and cause problems,bit seems that problem few and far between
Some fish will actively go hunting and eat them.
Imo I see them as beneficial part off system
Also edited above post a little before posted this ^_^
Aw just noticed you from north Yorkshire,I'm in Liverpool for last 15 years but originally from Bradford just over border on West Yorkshire ^_^
North Yorkshire such a beautiful place with North Yorkshire moors and Yorkshire dales