Hydroids are carnivorous animals, some are related to Jellyfish, and catch prey in the water column with the aid of stinging and grappling. Hydroids usually exist as colonies, either a branching structure attached to a rock or glass or it can have a more fern-like structure with microscopic polyps arranged along the individual branches. They can sting some corals and push out Zoas, so you need to control them.
A number of fish will feed on them, like wrasse and file fish. Also, you can use Kalk on them.