Army Chief of Staff Gen. Mark Milley was the keynote speaker at Norwich University during its Reserve Officer Training Corps Centennial Symposium, Northfield, Vermont, on April 21, 2016. During his speech he talked about "little green men and hybrid armies" [at 34m40s] This refers to forms of unconventional warfare carried out by Russia in the Ukraine. Little Green Men is a term used for anonymous forces who wore nondescript green military style clothing, (and were presumed to be Russian SPETSNAZ). The term was first used by the local Ukrainians, then used in academic papers: http://www.jhuapl.edu/ourwork/nsa/papers/ARIS_LittleGreenMen.pdf And the usage was quickly adopted: http://jsou.socom.mil/JSOU Publications/2016 Essays.pdf http://www.soc.mil/swcs/ProjectGray/Support to Political Warfare White Paper v2.3-RMT (10MAR2015) .pdf
Seems like by "hybrid armies" means quasi-armies like Daesh (IS) or other militarized groups like sect militias that are not necessarily national in origin
Hybrid armies or warfare I get, a known term. The little green men is one I never heard but could be a euphemism new to military discussion. I attended and completed Air War College in 1998.
I believe I have the answer. It is of Russian origin. https://en.m.wikipedia.org/wiki/Little_green_men_(Ukrainian_crisis)
Also called "Martians", which might confuse some: https://en.wikipedia.org/wiki/Little_green_men#.22Unidentified_defending_objects.22
The explanations come up quick if you restrict your searches to .mil (add site:.mil to the search terms) https://www.google.com/search?q="little+green+men"+hybrid+site%3A.mil
I have added explanations to the OP and will re-work it as a debunk rather than a question (changing the post owner to me). Thanks to @helenamatias for bringing this up.