Glaucus atlanticus

 (common names include the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small, blue sea slug.

 These sea slugs are  float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents.

 Glaucus atlanticus  is camouflaged: the blue side of their body faces upwards, blending in  with the blue of the water. 

The silver/grey side of the sea slugs faces  downwards, blending in with the silvery surface of the sea. Glaucus atlanticus feeds on other pelagic creatures, including the venomous siphonophore, the Portuguese man o' war. 

Humans handling the slug may receive a very painful and potentially  dangerous sting. 

For more infos about this sea dragon go to: 

https://en.wikipedia.org/wiki/Glaucus_atlanticus