Are these bad? I think it might be a hydroid

[Leaellynasaura]

A few have shown up recently in my coral quarantine tank.

 

[Dsnakes]

Yes I believe it is a type of hydroid. The ones I had on my glass like that eventually disappeared. I just scrubbed them off while cleaning glass and water changes. Never came back. I believe it's the colonial type that take over rocks.

 

[Mike N]

+1 ^
The colonial and digitate types should be feared.

 

[KJ]

Jellyfish?