Feathery retracting thing

I can't take a picture of this as it is on the backside, but I can see it from an angle. It is a white tube with a whitish, clearish feathery type thing on the end. The feathery thing sometimes retracts into the tube when my blue tang swims by fast, then slowly comes back out. Any ideas what it might be and if it's good, bad, or indifferent?

 

Just a small feather duster, completely normal.

 

Small feather duster. Very beneficial to the tank and fun to have!

 

They are good filter feeders.

 

Oh good. Thanks!