The Greys as we know of them in popular culture today were actually genetically-engineered servants working on a project to create a hybrid of humans and their original race. It’s unclear what the original Greys looked light because their sophisticated use of genetic engineering resulted in their society having many different sub-species. The “little green men” reported in the 1950’s were actually failed prototypes of the Huar species, re-purposed into continuing the project. “Failed” in the sense that they possessed the same flaw that was killing the Greys.
The Greys created the Huar because they were dying out to a plague, suspected to be a virus that targeted their base genetic code. Creating a hybrid with humans “patched” the genetic vulnerability. For the Greys, it was too late though. They died out before they could could supplement their populations with Huar and the empire died out with the few Huar left to fend for themselves. The Huar have since developed into a largely nomadic species, dependent on a number of colonies with no exact capitol or home planet.