Shiny Grimer might turn up in Pokemon GO more often if it is voted as the featured Pokemon this Community Day. Because Grimer has two different forms, it has two different shiny forms as well. Standard shiny Grimer is green whereas Alolan shiny Grimer is purple.
Both are more commonly found in cloudy conditions. Grimer's evolution Muk is incredibly useful as a tank, especially with the coverage offered in its Alolan form.
Shiny Grimer Pokemon GO: How to Catch
If Grimer is one of the winners of the poll you can expect it to have increased spawns during either the September or October Community Day. Because of the volume, Grimer's shiny versions will be easier to find during this time. Shiny Pokemon are also more likely to show up as part of raid battles, so keep you eye on gyms to try and catch a Grimer raid. If all else fails you can set up an incense or lure to further tip the RNG in your favor.