This is the sea slug Glaucus atlanticus, called the Blue Dragon.
Isn't it lovely? It gnaws on Portuguese Man O' Wars (men o' wars?), eating the nematocysts and concentrating the toxins in those many pointy dark blue frills. Look at them and admire their beauty, but do not touch.
The sneaky beasties don't just store toxins, they actually acquire functional nematocysts by eating cnidarians. This is a fairly common thing among nudibranchs.
I never knew these tiny things could sting you. And I would have thought I'd have found out, considering when you go for a swim when an Easterly has been blowing, you can hardly avoid them...
I've even picked them up - although very gently, of course, seeing as they look fairly fragile.