The last time I heard about "little green men" (other than ETs) was to describe obviously-but-unofficially Russian soldiers who invaded Crimea in 2014 and claimed to be natives who wanted Russia to annex the region.

https://en.wikipedia.org/wiki/Russian_occupation_of_Crimea

I guess it's now being used as pejorative against groups of vaguely military style. Like the DHS agents conducting Metro Surge.

ICE agents cosplaying as an army