In the Wikipedia article for the film Apollo 18, the rock-like spider creatures featured are often referred within the movie as 'aliens', but is that what they really are?
Are they aliens or could they be something else?
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Maybe I'm overthinking the premise of what is ultimately a cheap horror movie but I assumed they were some type of terrestrial primitive organism that came from the earth billions of years ago as spores.
My assumption was something like a highly evolved slime mold-lichen combination that managed to survive and evolve in the water rich subsurface of the lunar poles. There was perhaps a subsurface layer of moist black rock that made up a super colony.
The spider legs are actually tendrils evolved to handle the harsh cold vacuum while the core organism stayed safe in the rock. Sunlight would cause the tendrils to withdraw and the organism would become dormant until night. The spider forms would skitter over the surface looking for fresh subsurface water and organic deposits possibly from new comet impacts to nourish the colony or form new colonies. Primitive natural radio (like an electric eel) allowed the spider to keep in touch with the colony and help it to navigate.
Obviously a very hard and slow moving lifestyle. When the astronauts arrived it was dinner time!
Merriam Webster defines an alien as an extraterrestrial. If you lookup the definition of extraterrestial you get:
: originating, existing, or occurring outside the earth or its atmosphere
So by that definition, if they are not of the Earth then they are alien.
What are the alternatives?
A remotely possible scenario is that the creature were present on Earth a long time ago (but why are there no fossil records?), somehow got wiped out there (what could possibly make conditions on Earth less viable than the moon?) but a few fled to the moon (how?) and survived there, unable to return by themselves.