Eknodine

The Ecnodeen are a species of aliens that took over the bodies of old people in a dream created by the Dream Lord. The Doctor talks to them as if he knows of them, indicating that they do exist.

Physiology
The Ecnodeen are little green eye stalks that appear from their host's mouth.

Abilities
They could turn people into dust by spraying a sort of cloud over them. They could also use humans as hosts and allow them to live longer.

History
The Ecnodeen that the Doctor, Amy Pond and Rory Williams encounter said that they had ran from their home planet and came to Earth where they took hosts to stay inconspicuous.