Hydroids are closely related to jellyfish, but unlike the jellyfish, remain attached to the reef their whole lives. Like the jellyfish, hydroids can pack a powerful stinging punch. The sting from hydroids is considerably more powerful than that of most corals. They will tend to prefer your sps and lps and will grow over their skeleton; soft corals are less likely to be bothered because they do not provide ample space.

Hydroids filter feed and photosynthesis, which allows them to grow and colonize quickly. They prefer a lot of light and high water movement

There are two types of hydroids. The main one being colonial hydroids and less common digitate hydroids.

Solutions

Physical Removal -- Either picking them or burning them off the rock. Both are not good ideas. like aiptasia if you do not remove the whole entire hydroid root you risk the chance of these coming back.

Chemical Removal -Kalkwasser or super glue - you can smear kalk paste or super glue on these ... however, you chance that you won't get the whole tube emerged (remember these can get into small pores in the rock)  super glueing them you risk the chance of the hydroid actually rotting in the rock. This risks the chance of causing an infection to your nearby corals.

Natural Solutions

The lynx nudibranch (Phidiana lynceus) - Only eats hydroids, especially fond of Myrionema amboinense.

Pterolidia Ianthina - sea slug that feeds soley on hydroids

Sea Urchin Salmacis bicolor- Good eater of Myrionema. but very slow & can not get into the pores of the rocks where roots are.

Diadema setosum (Sea Urchin) - Good eater of Myrionema. but very slow & can not get into the pores of the rocks where roots are. Will also eat sps corals

Flameback Angelfish  (Centropyge acanthops and Centropyge loriculus)  - Hit & Miss and will occasionally nip at your corals especially your sps and some polyp corals.

Dolabella auricolaria - sea hare

Sources

http://en.wikipedia.org/wiki/Hydroid_(zoology)

http://www.seaslugforum.net/find/pteriant    

http://www.seaslugforum.net/showall/dolaauri

http://en.wikipedia.org/wiki/Pteraeolidia_ianthina
http://en.wikipedia.org/wiki/Diadema_setosum