The "Nightmare" aliens are a humanoid species which had developed a space faring civilisation as of the 20th centuryé The have a large head with a brain-like relief over their ehads, large bulbous round eyes and long point ears. A scout from this civilisation came to Earth in 1982 to test the human race through one of its specimen. It boarded a submarine carrying a large nuclear arsenal and tried to fire it in order to cause an all-out nuclear war. Success in stopping him was to be the proof the aliens needed to let Earth be while failure meant the destruction of the humans by their own hands. Luckily, the bomb were detonated before reaching detonation and the alien destroyed.
Source: Voyage to the Bottom of the Sea, "Nightmare", directed by Charles Rondeau, written by Irwin Allen and Sidney Marshall.