Alien Villagers are generated along with the Moon Villages, inside their huts.
They do not breed at all. Even if you place more wooden doors following the pre-1.8 vanilla villager method.
Baby Alien Villagers can be spawned only on creative mode with Spawn Eggs or via summon command.
Alien Villagers do have a trading system, as of Galacticraft 4. In Galacticraft 3 and previous versions they did not.
They all have the same trades to offer, but with some differences in prices so you can look around for the best price. Each villager ask different amounts of Lunar Sapphires for Oxygen Masks, Oxygen Gear, and partially filled Light Oxygen Tanks. They will also (for a price) exchange NASA Workbench schematics for equivalent level schematics, for example if you visited 2 dungeons and found 2 Moon Buggy schematics you can trade one of those schematics for a Tier 2 Rocket Schematic. If provided with an empty fuel canister and some sapphires Alien Villagers will fill the canister with fuel. Some villagers also trade ingots for their compressed equivalents
Attacking an Alien Villager within a village boundary does affect player's popularity points in that village. While this is irrelevant for trading, this makes any iron golems of that village aggressive to the "unpopular" players.
Like the vanilla Villagers, they will run for a shelter with a wooden door at night and during a rain storm, and stay indoors until the morning or clear weather. Since the Moon Villages are generated lacking wooden doors - and there is no rain on the Moon - this behavior is not noticed!
Evolved Zombies and vanilla Zombies do not attack them, nor do Alien Villager run away from zombies. On the other hand, as stated above, village's iron golems nearby will pursue the players that hurt alien villagers (only in Survival mode).
Killing them does not give any item or experience point.
They are tall and blue-skinned, with large noses and red eyes. They bear some resemblance to the villagers found in the Overworld, although they are taller.
They are about 2.4 blocks tall, so they will get hurt if put in a 2-block tall room. They also make the same noises as their vanilla counterparts.
As of Galacticraft 4, their heads are the same size as vanilla villagers, unlike in previous versions where their brains were far larger.
Backstory: Biology and Origin
Alien Villagers do not breathe – they live in an airless environment – and do not seem to have any mouths at all. Their brains, or at least their crania, are massive compared to Overworld Villagers, implying that they are highly intelligent. It's believed that they might have some kind of mind controlling brain power, as Evolved Zombies do not or cannot attack them.
These villagers are seen happily walking around the moon’s surface despite the extreme temperatures (as high as 390 K during the day and as low as 100 K at night) and its corroding regolith. Their robes also seem to be made of some material that can withstand such conditions.
It is unknown what moon villagers eat, as there are no obvious sources of food on the moon. They could possibly feed on the sapphires that they desire, or Cheese Curd, or they may eat the flesh of monsters they catch on the lunar surface. They may even be cannibalistic.
Alien Villagers have a deep fascination with trees. They are aware that the surface of the Overworld is covered with many trees, and it is a source of sadness for them that the Moon is not the same, so that trees have assumed an almost religious significance for them. They also revere all wood items. They are especially interested in Jungle Trees which they find magnificent. Their own attempts to grow trees on the Moon are generally unsuccessful - you can see saplings they have planted near their villages.
The origin of the moon villagers is a mystery. They may have evolved independently on the moon, or have been brought there by travelers from the Overworld. It is even possible that they were once brave colonists that evolved into what they are today. According to moon villager legend, their ancestors lived beneath earth’s oceans, meaning they may have evolved from highly intelligent sea creatures.