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.
Graham's Island 
Wow! First I thought it was a caterpillar. I don’t like slugs but this one is extraordinary!
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According to my marine invertebrates book, there are lots of incredibly colorful sea slugs. I just haven’t seen them, possibly because many are nocturnal and possibly because I haven’t been looking in the right spots. I shall pay more attention in future.
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And make sure you take a pic 😉
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