This is a blue sea dragon (according to the description, this was washed ashore). It is a nudibranch (Latin for naked gills) and is a sea slug. Nudibranches are known for their striking colours. This particular species floats in the open ocean (upside down because its stomach has a gas bubble to aid with the floating), carried by the currents and feeds on other similar open ocean travellers. It is about 3cm in size and is believed to be found throughout the world's oceans thriving in warm waters. If you happen to come across a beached blue sea dragon, don't touch it for it can give you a painful sting and a bunch of nasty symptoms. But how colourful they look!
By Sylke Rohrlach from Sydney - Blue dragon-glaucus atlanticus, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=39934058