Lichen

Lichen was found on a planet visited by the Seventh Doctor and Ace.

The Doctor discovered that the lichen had killed the human colonists who once lived on the planet. The lichen made its way into the TARDIS, causing many problems. When the Doctor and Ace went back to the planet, the lichen formed into the shape of a little girl and asked to be brought off the planet, but the Doctor and Ace left the lichen behind. (PROSE: The Ghost's Story ^[+]Loading...["The Ghost's Story (short story)"])

Tardigrades fed on lichen. (PROSE: The Short Briefing Sergeant's Tale ^[+]Loading...["The Short Briefing Sergeant's Tale (short story)"])

Tryods were known to eat at least some types of lichen found in caves. (PROSE: The Word of Asiries ^[+]Loading...["The Word of Asiries (short story)"])