Many have speculated on what Grey aliens would look like both inside and out. There are several schools of thought both from those who claim to have made contact with the entities and those who simply sit on the sidelines and speculate about what they think the creatures’ physiology is like.
One concept of the grey is that of the biological entity that we understand. It may have developed in a place with a far lower gravity, hence the large heads with disproportionately slender thin bodies. The interior of the ships obviously have some sort of gravitational support inside in order to keep the creatures comfortably light even in the denser gravitational area around Earth. The creatures are often said to possess incredible strength despite the fact that they are so thin possibly from strong muscles, but more likely from an advanced ability to manipulate momentum through the same gravitational technology that they utilize to provide them with a comfortable environment. They likely came from a place of much less light, hence the requirement of large eye sockets. Either they wear contact-type lenses over their eyes that allow the perfect amount of light to enter through the darkened shades, or their eyes naturally absorb more light. The second seems less likely since spacecraft are often reportedly brightly lit. They have a proportionately larger cerebral cavity than humans to their bodies suggesting an obvious heightened intelligence. The creatures noticeably never have reproductive organs, though they do find it functional to wear shiny clothing, suggesting there are still some elements from which the creatures find it necessary to shield their bodies even within their craft. Either that or they are simply noticeably modest. The small mouths are never used for communication, but telepathy is instead implemented. Telepathy, even in its most rudimentary form could result in a neural network between different members of the ship. On the other hand, deception and dishonesty would be difficult for species working within a complete hive mentality.
Alternately, the creatures could have been constructs designed ideally for space travel. With thin arms working in low gravity environments, the creatures would require little maintenance. Given limited instructions, they could be living out their preprogrammed routes for years, and with no further instructions they could have years left to fulfill their instructions without making a move. If the creatures were robotic constructs, however, why would they have mouths and nostrils? The sensory apparatus appear to be primarily telepathic and visual. Is the mouth merely there to create a machine in the image of its master? The automaton theory seems to be one that directly benefits the creatures visiting, as it suggests immortality. Therefore, it seems feasible that creatures could have included within some vessels robotic creatures that work on the same mental frequencies as the others, but are not biological like them.
What do such creatures eat? This brings up the more interesting notion of their limitations. They clearly have mastered deception and have it figured out to an art-form, but cannot create or destroy matter on the nanoscale. Otherwise, they wouldn’t have much reason to be here at all. Perhaps the answer to their motivation comes as a direct result to understanding their limitations and vice versa.