Colonial Hydroids

[Canadbis]

So How can I get rid of these buggers. They have totally infected the rock that they started on. I have removed that rock and placed it into fresh water and am now letting it dry out. BUT I have noticed a couple that are on the rock that was touching it. Do I have to do the same to the other rock or is ther another way I can get rid of it.
It is a reef tank and I have inverts and coral so I cant use anything that may hurt them.
TIA