Hydra's are small invertebrates that look like small tubes with tentacle like things extending from its mouth. and yes, they will eat small fry though they are not as horrible as they sound. most people who have hydras probably don't know because they hide in the gravel, decor or on plants. they are pretty picky of water quality so their existance in your aquarium means you have good water quality usually. if you do see a lot of them, your probably overfeeding.
ways to remove hydras include chemicals, copper meds, raising temperature, and dosing of aquarium salt but they are pretty harmless IMO.
do snails or shrimp eat hydra? Last week I had over a dozen on the glass wall and bare bottom. Then last sunday all but one disappeared. And today, that one is gone. Would snails suck them up thinking they taste like algae?