They aren't aliens.
From Wikipedia:
Methuselah (Vampire)
The Methuselah (based on the Biblical Methuselah) are a group of humans that have been infected by a virus that drives them to feed on human blood. Before the symptoms manifest, contact with sunlight does not harm them, and they age at the same speed as uninfected human. But once the symptoms start to make themselves evident, an "awakening", the virus will prolong the lifespan of its host, in addition to bestowing superior strength and healing ability. In turn, it feeds on its host's red blood cells, and when they become depleted, the virus will cause its host to crave blood, thus earning them the name "vampire". Common perceptions associated with the undead monster of mythology do not, however, hold true with regards to them: they can procreate (offspring inherit the condition), and they can consume normal food.
Once exposed to UV radiation, the virus will become overactive and consume the host's body. This gives the impression that the Methuselah's body is being burned up by sunlight and smoking - the latter effect being, in truth, water vapor.
The term "Methuselah" in the context of the series is what most vampires in general and Imperial nobles in particular prefer to be referred as; to call them "vampires" to their faces is often construed as a vulgar insult.