Is it good or pest?

[Alex2002]

Hello!

A couple of weeks ago, I started to get these white things on my stones that release these whip-like tentacles at night (as you can see in the video).
Are these things harmful to the tank inhabitants or can they stay? And what kind of creatures are they anyway?

Thanks in advance for any help!

 

[kchristensen8064]

Your video isn't working for me.

 

[Spare time]

Hydroids. They can sting corals but I don't think there is anything you can do to remove them.

 

[Alex2002]

Your video isn't working for me.

Here is a pic.

 

[the bomb]

That’s a pest