What is this? Algae? Hidroid?

[Monover]

I have these white stuf growing in some frags, under the rocks, etc.
What do you think?
They grow fast, if a clean a frag with the teeth brush, in a few days grows again. In zoanthids frags i was doing peroxide hidrogen baths and the same, in one week they are again.
Any idea?
Thanks!

 

[Auquanut]

Looks like hydroids.

 

[Monover]

Thanks! It was my first option, but with your opinion, i am 100% sure..