Asap unknown thing

[Tropical234]

i have this live tiny “thing” that was on my rock that i have no clue what it is. It moves and contracts when i had the fish net near it. I git it off and out but i fear it probably escaped because of how small it is compared to the wholes in my net. I also am scared the fish might of eaten it. What is this and is it safe in my aquarium

 

[Autumnsky]

Hydra. Kinda like a jellyfish with an attachment, and can sting.

https://en.wikipedia.org/wiki/Hydra_(genus)