This is Glaucus atlanticus, a sea slug found in tropical and temperate waters throughout the world. These photographs make the rounds on Facebook every few months simply because they’re so strangely beautiful.
However, no one ever mentions that this sea slug (like other sea slugs) is a total badass. Why, you ask? It eats incredibly venomous animals like the Portuguese Man o’ War. The sting is agonizingly painful to a human and to most animals, but poses no problem to Glaucus atlanticus, which is immune. After the sea slug has consumed the Portuguese Man o’ War it “selects” the most venomous cells for its own use. The slug then incorporates these cells into its own tissues, and uses them as a defense mechanism. It also concentrates the venom, making it far more venomous than the animal it “stole” the cells from.