Dee aliums. what I think they should be
the Aliens are a Cunning but overzealous race. they have a humongous following for their own religion which is believing in a Sentient Power generator that generates trillions of Electricity for the Aliens.
however in order to keep the "Generator" Generating they pick a planet and start Secretly abducting the inhabitants. in return the Aliens protect the planet from "rogue" Aliens. However the "Generator" creates a massive amount of radiation so the Generator is stationed the "Abductee" planet so the Radiation can be safely stored on the planet.
but however after 50 years the entire inhabitants of the planet will die of radiation and their will be enough power to power the Aliens homeworld for a century. and then the Cycle will happen again.
the Aliens can disguise themselves as any member of any species. including humans. they're main purpose on Earth is to Abduct people and sacrifice them to the generator. they're also using Earth as a place to store the pollution from the generator.
Its basically the Incubators from Madoka magica. and the techpriests from Warhammer 40k. thats my 5 cents. don't spend them on a pencil @ArenaSnow
I see no problem with the idea. :)