These slugs are known to biologists as Glaucus atlanticus, and they're not frequently seen because they spend their lives floating through currents far out at sea, as The Dodo reported.
Facebook user Lucinda Fry documented her rare up-close look at one in the video below:
Blue dragons are generally smaller than one inch in length, and they're full of poison because they feed on Portuguese man-of-wars—you know, those floating terrors (also known as blue bottles) whose tentacles can be up to 30 feet long.
The blue dragon consumes the stinging cells that the man-of-war uses to immobilize fish, but instead of digesting the toxins, it stores them in sacs on its outer appendages.
According to a scientist interviewed by Australia's Gold Coast Bulletin, it's a good thing that Fry didn't try to touch the slug she saw, because it might have stung her.