The Blue Dragon (Glaucus atlanticus) from the family Glaucidae is a stunning nudibranch that we see in Sydney occassionally.
This rare type of nudibranch is only seen in Sydney after strong swell, sea currents or winds bringing to shore large numbers of hydrozoans (e.g. Blue Bottle jellyfish and Blue Button jellyfish). This pelagic nudibranch lives in the open ocean, travelling upside down on the ocean surface, surviving on an air bubble which it swallows and keeps in its belly, whilst letting the wind help it chase the hydrozoans upon which it feeds.
They are the size of a coin and truly magnificent. But be careful not to touch these nudibranchs as they retain the sting (nematocysts) they receive from digesting the Blue Bottle jellyfish. Their sting is far more potent than the jellyfish and can be potentially dangerous to a human so if you ever get stung immediately call the emergency services as the sting has led to shock, fever, heart problems and has also been known to be fatal.