Orange Fireworm (Eurythoe complanata) ハナオレウミケムシ also Bristleworm
The Orange Fireworm hides in holes or under rocks in the day. They come out at night to feed. It was once considered to be a worldwide species, but now some believe it to be at least three separate spices. The bristles are what give it the fireworm part of its name. They are very sharp and come off easily.
Another picture of the same Orange Fireworm. They can also move quite fast and even swim.