Hydra are a type of freshwater coelenterate related to marine anemones. They are usually found in ponds, but are sometimes introduced with plants or livefoods.
Hydra are predators and usually feed upon small zooplankton like Daphnia. Their bodies and tentacles are equipped with tiny harpoon-like stinging cells called nematocysts with which they spear passers-by. They rarely do large fishes any harm, but can sting and catch very small ones.
Hydra are sensitive to copper, so you can eradicate them with a copper-based disease medication. They tend to thrive in dirty tanks.
I'd suggest moving your newts to a hospital tank while you treat the community tank with the copper-based medication. The only way to remove copper from the aquarium is with EDTA (a chelating agent) and water changes. Use a copper test kit and when the copper is gone, reintroduce your newts.