From your picture, this is a type of mosaic jellyfish.

The mosaic jellyfish is a marine organism of the order Rooted Mouth Jellyfish of the order Potamonoidea. Also known as the blue blubber jellyfish, the main stage of the life history is the monotreme, jellyfish type, with its hydroid stage underdeveloped or completely disappearing.

The jellyfish body is different from that of the Hydra, and this difference is mainly manifested in the fact that its jellyfish body is generally larger in size and has no rim