If it’s green and fizzy in Japan, then it’s not lime flavour, it’s Melon soda. Specifically if you’re at a McDonald’s then it’s a Fanta Melon soda. McDonald’s throughout Asia have long offered ice cream float options on their Coke and fruit flavour sodas (known as McFloats in Philippines and Singapore, and occasionally in the USA), where a big squirt of McFlurry ice cream (without the bits) is squirted on top and in Japan you can have that in Melon or Coke flavours. Japanese melon soda is not necessarily easy to describe, tasting as it does ‘melon-y’ without being readily identifiable as a particular kind of melon. You can also pretty much guarantee that Fanta Melon soda* has not been anywhere near any real melons. But that doesn’t stop it tasting great.
Ice Cream Floats are great. Especially in Asia’s 35+ degree summers. But they’re great at any time too. Seriously McDonald’s, why don’t you do this in the rest of the world?
*Other melon soda brands are available. No melons were likely harmed in the production of any brand of Japanese melon soda.