This colorful little creature is a sea slug. As with many other nudibranchs, the bright coloring serves as a warning. They’re poisonous and, because of this, have no known predators. One tidbit I found interesting is that nudibranchs like this Varicose phyllidia, don’t produce their own poisons. Instead, they ingest toxins from their prey, such as sponges, and recycle it. So not just yellow and black and pale blue, but green as well.