Most sea slugs crawl on a substrate. Glaucus atlanticus lives in the neuston, the thin film of life at the ocean surface, and has no substrate at all. It maintains position by holding a bubble of swallowed air in a specialised stomach sac, which keeps it at the interface, and it drifts wherever wind and current carry it.
The animal floats with its dorsal surface downward and its foot toward the sky, so the usual countershading is inverted relative to body anatomy. The blue that faces upward hides it from seabirds against the water; the pale silvery-grey that faces downward hides it from fish looking up into bright sky.
Its prey includes the Portuguese man o' war, and like aeolid nudibranchs it stores the prey's undischarged stinging cells in its cerata. Because it concentrates them, a blue dragon can sting a human more sharply than its size suggests. The common claim that it is more venomous than the man o' war is not established; what is true is that it delivers borrowed venom, and it should not be picked up when it washes ashore.