Beaches in New Jersey have seen jellyfish-like critters are commonly known as blue buttons (Porpita porpioat), but they aren’t natives to the state.
Although they look like jellyfish, they are a different set of creatures, known as a “ type of creature (or rather, set of creatures) altogether. They fall into the scientific class Hydrozoa, whose members are each made up of colonies of hydroids — tiny predators that are related to jellyfish. The most famous hydrozoan is probably the Portuguese man o' War (Physalia physalis), which can deliver a venomous sting so powerful that it can kill fish and even injure humans, according to National Geographic.”
These Portuguese man o' Wars can cause considerable trouble to swimmers. As the fall in Geneses 3 promised, they can be hazardous.
However, the blue buttons (Porpita porpita) can not so dangerous, but caution is the safest bet.
Geggel, Laura. 2018. Rare, Blue Jellyfish-Like Creatures Wash Ashore in NJ, Puzzling Beachgoers. Live Science (4 October).