Oog-Oh, a Planet Jacker

Planet Jackers are an alien race who hail from planet Guarven. They were featured in the  episode of the same name. Due to the fact that their planet's star is dying, they steal or "jack" (hence their name) other planets and throw them into the star like "firewood", as Zim put it. They thus are responsible for the mass genocides of several interplanetary species. They were shown to have advanced planet-moving technology.
Planet Jacker 6

A Planet Jacker, showing its entire form.

Planet Jackers have red tongues and saliva that appears to be of a normal hue, although since they are always seen with small tubes entering their mouths that resemble dental rinse tubes on Earth, it could be that they must constantly keep themselves hydrated and this "saliva" is in fact the fluid they douse their mouths with. Larger Jackers seem to require more hydration, although this could also be a personal preference. Their race are not above cybernetic enhancement and while they naturally possess three fingers and an opposable thumb on each hand, their cybernetic replacement hands instead only possess two finger-like appendages.

Planet Jacker skin can range from an asparagus color to a more pine green hue. Their eyes seem to be consistently green-yellow. Ironically, despite being fully green themselves, they still referred to Zim upon encountering him as "that green guy".

Irken-Planet Jacker Treaty

The Planet Jackers have a treaty with the Irken Empire; this would make them one of the few races that the Irkens are not directly trying to conquer, possibly due to the threat from their advanced planet-jacking technology. The treaty restricts the Planet Jackers from jacking planets marked for Irken conquest.