Or my working theory.
Other older civilizations. Ended war. So when there is a disputed planet or civilization. They each send small team to establish their belief system. After an agreed amount of time. They count up followers whoever has the most wins. Like proxy wars. They then cull the planet.
Jewish energy beings
Buddist Hindu insectiods.
Greys are just genetic hardware that goes with the craft they are used like probes quantum telepathy hardware and genetics make them proxies they have modified telomeres in their DNA the Grey's age really slow they fly the ship outside the time bubble that prevents the occupants from aging the 40 or so years they warp to get here.....