Moon jellyfish

The Moon Jellyfish is a sea creature in introduced in the 1.3.0 Free Summer Update.

In
When donating to the museum, Blathers will say the following:

After being donated, it will be visible in a large swarm under a floorglass by the Sea Butterfly's tank.

Real-world information
The moon jellyfish is almost entirely translucent, usually about 25–40 cm (10–16 in) in diameter. It feeds by collecting medusae, plankton, and mollusks with its tentacles, and bringing them into its body for digestion. It is capable of only limited motion, and drifts with the current, even when swimming.