Alola is both a global tourist attraction and one of the most isolated places on Earth. Although a select few trainers arrive on the backs of their pokemon, the overwhelming majority of visitors arrive by boat or plane. Yet the fuel consumed by these vehicles has polluted Alola's harbors and coastlines, especially when accidents release a trans-pacific journey's worth of it in one place. So dangerous is the pollution to this famously beautiful land that the Kahunas of Melemele and Ula'Ula island imported Grimer from Kanto, in the desperate hope that they'd devour more poison than they'd create.
Although defenders of the program claim the new Grimer reduced the amount of garbage circulating around the islands as a whole, Grimer clearly failed at their primary task, as the beaches of Hau'oli and Maile city remain far too dangerous for swimming. Yet tourists once again flocked to those highly toxic ports, not to swim but to witness the rare beauty of the Alolan Grimer.
There is a certain appeal in the coloration of an oil slick, and a Grimer's whole body could be fairly compared to a stomach, with little distinction between what they are and what they devour. In Celadon City and other places around the world where they're born, Grimer are typically the grey color of industrial sludge, and only rare individuals eat enough fuel to turn a brilliant green. But in Alola, this is the typical appearance of the Grimer, along with a hint of a rainbow around the mouth which develops further after evolution.
One can find clean, sunny beaches anywhere, and the residents of Hau'oli and Maile will gladly travel to other towns to enjoy them. Their economies still boom with tourists, for only they can claim the presence of the Alolan Grimer!