Are these harmful? ID please.

[jsnmldr1]

 

[KING KONG]

Hydroids,known to sting fishes. I too hv and they reproduce at a very high rate. Mine never harmed so I let them be there

 

[ISpeakForTheSeas]

Agreed with hydroids:
I've heard smothering the hydroids with kalk paste may work, but here are some other threads with suggestions that might be helpful:

Hydroids: a reference to some strategies

Like many of us, I have had problems on and off over the years with Hydroids. The ones that have always given me the most problems are the common ones that almost look like a smaller and darker version of the beginner coral called Yellow Polyps. But the colonial Hydroid is much smaller than...

www.reef2reef.com

Hydroids - How do I eliminate- They have taken over my tank

So I have this stuff that has pretty much taken over my tank and I can't get rid of it. I'm looking for an ID and an eradication plan that is anything short of a total teardown.

www.reef2reef.com