It's actually called Mizu Shingen Mochi but since it's a weird, tasteless gelatin blob it's being marketed to US customers as a "raindrop cake". It's a common desert in Japan, made out of a seaweed based gelatin called Agar. Since it doesn't taste like anything, it's usually accompanied by a sweet syrup and a soybean powder called kinako.
You can even learn to make it in this video:
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