Hard white things

[dromo2189]

Wondering if someone knew what this is and if it's harmful?

 

[melosu58]

Looks like a cluster of mini dusters.

 

[dromo2189]

Are they harmful? I know they been on that rock for about a year

 

[melosu58]

There are worms in those cases. They are good filter feeders. They might not be there any more. You can tell if there crowns are out of the tubes.