They spread really fast. Do not try to pick them off or they float around and anchor to other rocks. You will then have a bigger problem. The only thing that I found that will eat them is chocolate chip starfish, but the starfish also ate my zoos and shrooms.
I have been battling these hydroids for years and can't get rid of them. I am currently running my tank with Zeovit and have reduced the nutients in the tank significantly. This seems to limit the spread of hydroids.
Jim
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