Our condolences for the Pancham that got dissolved by the Alolan Muk's Acid Downpour.
The Tsubasa Tokyo Revelations version of Tokyo was often nicknamed "Acid Tokyo" due to the city being reduced to ruins by acid rain. Also, Acid Downpour is a Poison-Type Pokemon's Z-move, so Tokyo turning into Acid Tokyo due to a Poison-Type Pokemon such as Alolan Muk using Acid Downpour just to defeat an enemy Pokemon seems to be a logical origin story that makes sense.
Pokemon Sun and Moon © Nintendo/Game Freak. Tsubasa Reservoir Chronicle © CLAMP/Kodansha.