Any idea what this is? It's about 3-4mm. It retracts into the rockwork periodically.

[bashierr]

y i

 

[bashierr]

I have found two of them in my tank

 

[Goaway]

Feather duster. Many species. Reef safe and harmless

 

[bashierr]

Thanks!

 

[vetteguy53081]

Not feather duster- Its a tube worm and uses its fan to capture food and favors Brine Shrimp. Can colonize.
Place a dab of gel superglue over the opening and it will starve