Some help with hydroids please?

[detroit_fan]

I believe I have colonial hydroids and need to get rid of them.

Since I have so many, I think I will just take the rock out and let it dry, too many to try and get with a torch or kalk paste.

How long will the rock have to sit dry before they are dead?

Also, can these things spread through the water column or do they have to be touching other rock to spread?

Thanks