SLENDERS

Slenders are a species of tall circa 4 meters tall grey humanoids who developed an advanced space faring civilisation. They are an intrusive species, harnessing other species such as  skitters  and humans to control them and armed Mech drones to ensure their security. Graceful and slender, they can move very quickly in an Earht type gravity and appear to have features ressembling cetacean mammals. They are the apparent power behind the invasion of Earth. 

Source: Falling Skies