Actually there are a number of lovely sea-slugs, but Glaucus Atlanticus has special appeal. They are pelagic (they really travel) roaming the world's temperate and tropical oceans feeding on Men-O-War and other floaters, even though they're barely more than an inch long. An air sac in their stomachs keeps them afloat. They have a fascinating life history which you can easily look up.
Unfortunately, they do poorly in captivity.
In the pictures, you're looking at their ventral surface---their bottom---which faces up in life. The dorsal is a uniform silver-blue---and faces down.