This is Spirocodon saltatrix. It is a hydrozoan, known as kami-kurage (カミクラゲ) in Japanese. They are found along the Pacific coast of Japan, often within harbours.
Kurage is the word for jellyfish in Japanese. The prefix kami means hair, a reference to the hydrozoan's hair-like tentacles.
Though closely related, hydrozoans aren't jellyfish. But they sting, which is the part that matters. In this instance, the venom isn't terribly strong and there are few if any reported cases of stings.
This would be unlike the more notorious hydrozoan Physalia physalis, aka Portuguese man o'war, whose stings hurt in a major way.
Trust me on this. I've been stung (too) many times.
Here's a fun fact for trivia night: See the red dots at the base of each tentacle? Those are light-sensing organs. They can't see per se, but these animals are aware of changes in luminance levels and no doubt make use of that capability to structure their activities.
They have, in a sense, seen the light.