What you see in the palm of this person's hand is a “piece” of Water Cake. And if you can follow the instructions provided in the YouTube video, you'll actually be able to experience not just making it, but also finding out what Water Cake tastes like.
The Nerdist was the source for this bit of information and has a set of photos to view.
They explain about this Japanese dessert:
I give you… the water cake. To the naked eye it may look like the return of Clear Pepsi or a giant bubble of water, when in fact it’s a transparent piece of Japanese rice cake commonly known as Mizu Shingen Mochi. The best, most futuristic part about this dessert? If you don’t eat it within 30 minutes it disappears.
If you must try it, there are only two choices, travel to Japan and hunt it down, or try your hand at making your own.
News to Share brief source: “THE FUTURE OF DESSERT IS HERE, AND IT’S A WATER CAKE!” by JENN FUJIKAWA on Nerdist
Image still from the YouTube video “Mizu Shingen Mochi Recipe” by: Hey! It's Mosogourmet!! あっ、 妄想グルメだ！!
YouTube video description:
This looks like a large drop of water but it’s actually a cake! It's called Mizu Shingen Mochi and it's so delicate it needs to be consumed in less than 30 minutes, after which it will turn into a sweet puddle of water!