This is a Flower Hat Jelly. They are found off the coast of Japan, but there have been sightings in Brazil and Argentina.
Flower Hat Jellies eat fish and aquatic invertebrates. They catch them in their tentacles. This kills the animal. Then then eat it. It will only leave a rash on people. However they can have a population peak in seemingly random times (called blooms). Then they may be harmful.
I saw this jellyfish at Shedd Aquarium when the Jellies exhibit was there.