purple “feathers”

[Corlexplsion]

I have an established 90 gal reef tank with a few fish. All of a sudden I’ve noticed two small bushels of dark purple feathers pop out of the live rock. They don’t respond at all when touched. Any clue what these are and if they are bad or good? Thank you!

 

[Ranjib]

Like they keep their crown open even if you touch .. thats seems very unusual . I dobt its a feather duster if its not spooking out