The best approach is to remove the rocks and physically remove the hydroids. You can use peroxide to kill them on the surface of the rock and then scrub the rock clean in a container of saltwater. Some hobbyists have had success using Frank's F-Aiptasia in the tank directly for mild infestations.
yeah you have to scrub your rocks or legit just bleach them to get rid of them. though ive had tanks with them and never had them bother any corals or live stock. ive even had some colonies that glow solid green that look pretty nice in the tank.
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...