ID this growing on mushroom

[Dimmy007]

Hello, looking for help. These feather looking things growing on the mushroom coral. Good or bad. Should I remove. Is it part of the coral.
Thanks in advance.

 

[sbash]

Looks like a common cespitularia or possibly anthelia (depends on the stock). If not kept in check it can be come a nuisance. Like xenia, you can probably carefully peel the foot off the rock. I use a pointy set of metal tweezers.

 

[VSVP bet]

Unsure of the official name, but a long polyp xenia species none the less. Grows very fast, id recommend keeping it on an isolated rock so it does not spread like crazy, or just removing it all together, your call.