So the grey starts the discussion. Low muscle density and large eyes indicates a small low gravity planet with a lower light level. Less gravity nedds less muscles, and could explain the stck figure with big head idealization of the grey. The big eyes do indicate very low light levels, and may have photo receptors for other light wave frequencies. The colours red, green and blue are within us because of heamoglobin, clorophyll, sky and water. Life could be based on other colours, by use of different metal pigments other than iron and magnesium. A lower incident light could mean a bigger star with the planet further away from it to be in dusk. Kepler's law would indicate a longer year.
The water balance on a earth like planet would be crytical. Too much and the desity of the seas would slow motive evolution, too little and micro-ecology islands would develop, with much drought time. The concentration of salt in the water also limits fresh water life. A moon of a large planet could support liquid water if the green house effect was suitably big enough to keep heat in during the transit eclipse of the star. Water makes an excelent biofluid due to high specific heat capacity for thermo regulation. It also has a relatively high evaporation energy, limiting the work needed to maintain body homeostatis. Ammonia could be a replacement for water at a lower temperature, but acidity and alkalinity would be judged by different standards. Lower atmosphere pressure due to lower gravity will slightly decrese melting and boiling points of water and ammonia.
The above does assume that the greys are not genetically enginerred to be vessels of the true greys, but with adaptations for our environment. If they are just drone environment clones this could explain there desire for local DNA supplies.
Given that quantum information transmission has been demostrated here on earth, communication at any distance is possible. A common star, masar pulsar or optical source would form the entanglement distribution centre, with local reception of statistically entangled pairs. I do not see how this could not be used as a galactic internet backbone.
Relativity and space travel comes with a large potential for time dialation. This would of course lead to beings returning to origin in quite 'last years' model spaceships. Any species of humour would of course find this amusing and possibly lead to short lived conflict and possible environmental regulations being broken.
I wrote about this idea on the virtual medicine page.