The Leech
A visitor should be fed, but this one could eat you out of house and home ... literally!

A tiny spore that looked like nothing more than a speck of rock dust drifted through space for untold thousands of millennia and by chance passed close enough to earth to be caught in it's gravitation field and gently pulled down to land somewhere in New York State, USA. When discovered, people called it the Leech because it fed on radiation, soil, rocks, and anything that came into contact with it's grey surface. It fed and it grew. And grew. Every attempt to blow it up or burn it away simply fed it more quickly. And it continued to grow. The Leech became conscious when about the size of a car, a very flat car. It was then that the Leech became really serious about the need to eat and grow.