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 aliens inside of human hosts. Only the eyestalk is seen.

Abilities
They could turn people into dust by spraying a green 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 were driven away from their home planet and came to Earth where they took hosts to stay inconspicuous.

Behind the Scenes
It is possible that the thing that has driven them away from their planet is the Silent Menace as this has also driven the Saturnynians from their home planet to Earth.