No seriously, science side of Tumblr, we need you
It’s carbon fibre film. Carbon fiber is basically very thin strands of carbon — even thinner than human hair. The strands can be twisted together, like yarn. The yarns can be woven together, like cloth. To make carbon fiber take on a permanent shape, it can be laid over a mold, then coated with a stiff resin or plastic. this film is on water so when the guy puts his hand in and swishes it around he’s basically wrapping the film around his hand.
This is referred to as hydro dipping. It is commonly used on car parts such as spoilers and hood scoops. :)
They actually make this is tons of different patterns, everything from camo to wood to stone. You can buy your own kit for under 100$. It can be used for putting patterns on anything from props to bike helmets. Great for cosplay.