What is this ? a feather dust?

[majid kalhor]

Hi , i have a new 360 liter tank . i see today this on my rocks .its very small and i cant take a good photo of it .if a fish or something go near it ,it quickly retract within.can someone tell what is this ? is it harmful or not?

 

[Tyler_Fishman]

yes, feather duster worm, harmless