I have had to deal with a different type of hydroid in my tank as well. They do eventually die on their own in most tanks, but it is possible for an established tank to keep feeding them and allowing them to reproduce. The hydroids i had in my tank were continuing to breed to the point where i was getting stung all over my arms any time i reached in my tank. What i did, and suggest, is to suck as many of them out with a turkey baster or something similar, if their "string" doesnt come loose you can just snip it with some scissors or something and then suck them all out with the baster. I did this daily and it took about 2 weeks of this for them to stop breeding and die.