Weird Creature Appeared In Tank

Google hydra viridis and see if they look like your invaders. There are a few different species of hydra, yours might not be viridis but the all look more or less the same.
Not a bad invader though they have been known to sting and kill small fry. Adult fish are too big to be harmed by them. Some fish will even eat them.

There is a way to get rid of them though I wouldn't use it lightly. Wormer plus, the discus med, kills them but it also kills snails and possibly shrimps. And if you can live with them it's better than dosing the tank with a powerful medication.

I had hydra in my tank several years ago, I recognised them from biology lessons. That's how I know about wormer plus, and how I discovered that it kills snails even after using carbon to remove the med