This is Nembrotha cristata, a colourful sea-slug found in the tropical Indo-West Pacific Ocean. They grow to about in length and have black bodies with a strange luminous green "trim". They deliver a painful sting.
Sea Bunnies: Japan Is Going Crazy About These Furry Sea Slugs
freespiritmuse: “ Mesmerizing Chandelier Mimicks Deep Sea Bioluminescence The bioluminescence of deep sea organisms is a magical thing to behold. Czech designer Kateřina Smolíková captures this wonder.
Larval Leaf Scorpionfish (photograph by Chris Newbert, Minden Pictures). Lacking any other defense, many larval fish have adapted transparency as a method of camouflage—such as this tiny, see-through larval leaf scorpionfish in Hawaii.