Lunarians are technically not aliens; rather, they are a human subspecies. Humans colonized the moon long ago and deliberatly altered their genes in small ways to make living on the moon easier. Eventually, they changed their genes so much that they could go outside without spacesuits.

Lunarians are dependent on imports from Earth (fortunately they have a very good relationship with Earth) and other planets since the moon is still otherwise lifeless. Lunarians are developing genetically engineered plants that can survive on the moon, but the moon still can't provide water for the plants or the people. The moon does have trace amounts of water ice at its poles, but this is not enough to sustain a civilization.