Dusty was the name given to an energy creature that lived on the Moon. He took the form of a dust storm.
He had been stranded far from home for thousands of years. When three astronauts came to the Moon, Dusty tried to communicate with them, but had little success, the astronauts seeing his attemps as an attack. The Sixth Doctor and Evelyn Smythe arrived at the moonbase, and the Doctor was able to communicate telepathically with Dusty. Once the astronauts learned his true intentions, he was invited in for Christmas, and took an approximately human form.