I have read that some Limpet types and a specific Nudibranch may eat Hydroids, other than that I know of no other common predator.
If the Hydroids are limited to one small area, you can remove that pieces and treat the area out of the tank. If you can not remove the piece the Hydroids are on you can take a piece of underwater epoxy and flatten it over the area of Hydroids, leaving the epoxy in place for a week or so.
Not all Hydroids are created equal but most should be removed ASAP as they tend to be extremely invasive.