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)