What is this? Feather duster maybe?

[Twalker791]

Hello! I bought some dry live rock and am in the process of fishless-cycling my tank. Today I just noticed this little guy and can’t quite seem to figure out what it is. A feather duster of some kind maybe? Hopefully nothing bad!! Thanks!

 

[Biglew11]

Mini featherduster good guy.

 

[vetteguy53081]

Its a calcified house holding a fan worm whichis a harmless filter feeder