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.
Currently they do not have any trade system.
Attacking an Alien Villager within a village boundary does affect player's popularity points in that village. While this is irrelevant for trading (as non-existent), this makes the 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, and stay indoors until the morning or clean weather. Since the Moon Villages are generated lacking wooden doors, 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, red eyes, and massive brains. They bear some resemblance to the villagers found in the Overworld, although they are taller and cannot be traded with.
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.
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 trees seen growing in their villages, or they may eat the flesh of monsters they catch on the lunar surface. They may even be cannibalistic.
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 in tiki-heads beneath earth’s oceans, meaning they may have evolved from highly intelligent sea creatures that worshiped gods and behaved like under water tikis.