- "These cells duplicate faster than anything known to man. Their growth rate is incredible, in fact it’s frightening."
- ―Dr. Hans Halvorsen
The Green Slime, as its nickname suggests, was originally an amorphous green-colored sludge-like lifeform which was discovered inhabiting the asteroid Flora, which was moving towards the Earth. Strangely for such a celestial body, the asteroid seemed to have Earth-like gravity and shallow water puddles scattered on its surface, alongside which the green creatures grew like patches of seaweed. They showed limited mobility, inflating and deflating rhythmically and being able to crawl around the ground, and also had a faint phosphorescent glow. While Flora has been since destroyed by explosives planted by a team of astronauts led by Commander Jack Rankin to prevent collision with Earth, a small sample of one of the creatures' tissue was accidentally brought on board the space station UNSC Gamma 3, attached to one of the astronauts' uniform.
After being exposed to the energy of the decontamination chamber, the substance quickly grew and mutated, giving rise to a human-sized cyclopean creature, dark green in color (and with light green blood), with a huge single red eye and what appear to be multiple smaller eyes on its chest as well as very long tentacles terminating in red chelae. Most dangerously, it possesses the ability to release strong electrical discharges with its tentacles, which electrocute victims immediately. The creature has an incredible rate of cellular growth, as noted by Dr. Halvorsen, and is able to heal its own injuries quickly as well as generate a whole new organism from even a single cell. It can also feed on many kinds of technologically-generated energy (as well as sunlight), which renders it nearly immune to the Gamma 3 crew's weaponry.
Once the original beast was injured, a few drops of its blood turned into a vast puddle in a matter of seconds, and from that multiple new creatures emerged in merely a few minutes. Their level of intellect is unknown: they appear to be non-aggressive towards each other but highly hostile to humans, and make squeaking-like sounds, possibly as a form of communication. They are also able to survive in space vacuum.