Which hydroids are you referring to? Colonial (sessile) or hydra (motile)?
Hydroids typically bloom due to an abundance small, usualy live, food items. The hydra typically just go away on their own, the colonials don't really hurt anything unless they're in proximity to a sensitive coral. None of them can harm fish, with the exception of fry.
You can get rid of them by treating the tank with H2O2 or with Fenbendazole, but inverts don't like the treatment very much.