Hydroid looks like a fan worm, but smaller and has a soft tube. It usually has a brown color, under actin light it may glow green. They are both filter feeder and photosynthetic animal. So the grow in the tank fast and can overtake your tank.
At first, Hydroid may look cool and innocent, but it will sting your coral ,fish and yourself. Hydroid should removed as soon as you find it. Always check your new coral before put in your tank.
Even using a twister to pull the Hydroid out is not effective, it will retract to the hole immediately when you touch it. The only effect way is to burn it or cook it.