They should be fine as long as it's not an infestation.
Hydra's stinging cells are meant to paralyze their prey so they can eat them.
Luckily for you, their native food is daphnia and cyclops. (about the size of this period- . )
The fish should have the sense to avoid them.
I just did research, and your fish should be fine.
If you notice any symptoms, you can kill them with 2ml of 3% hydrogen peroxide per gallon, or pretty much any anti-ich (or anti-parasite?) medication.
If you happen to have a syringe, you can apply peroxide or salt locally to kill them. manual removal might not work too well.