"Patient Zero" (Sleep No More)

"Patient Zero" was a human who became the first "client" of the Morpheus pods. Throughout his long sleep, so much sleep dust had formed in his eyes that it had completely enveloped him and transformed him into a Sandman. He emerged from his pod when a reanimated Gagan Rassmussen released him on board the Le Verrier space station, and was set loose on the Twelfth Doctor, Clara Oswald and Jagganth Daiki-Nagata, but crumbled away into dust when the Doctor reversed the Le Verrier's gravity shields and escaped in the TARDIS. (TV: Sleep No More)

	Sandy
	Human, Sandman
	Gagan Rassmussen
	Triton
	Sleep No More
	Paul Davis

You may be looking for the medical term or the audio story.

Abilities

The Sandman had the ability to control the other Sandmen, commanding them to surround the TARDIS and keep the Doctor and Clara out. It could also turn into dust at will but was weakened by the gravity shields. (TV: Sleep No More)