Emojibot

The Emojibots or emoticon robots (PROSE: Diamond Dogs) were humanoid robots that served as an interface to the microbots known as the Vardy. As Bill remarked, they spoke Emoji — one form of language which lasted in human culture long into Bill's relative future.

The Emojibots as well as the Vardy were made to keep all of the colonists happy at all times. When a woman died of natural causes, her family and friends began to grieve. The Vardy read grief as an enemy of happiness and as a result killed anyone experiencing it. Eventually, all of the colonists who were awake at the time were killed, their skeletons used as fertiliser.

The Twelfth Doctor and Bill were handed mood badges so that it was able to keep track of what their emotional status was. It was able to read when Bill was hungry and fed her algae. When they found out that the colonists were murdered, the Emojibots read that they were upset so began chasing them and surrounding them in order to kill them. The Doctor found out that by smiling and becoming happy the Emojibots would let you go.

When the remaining colonists woke up, the Doctor explained that the few colonists awake before were murdered, but before listening to him completely they took up arms in order to fight the Emojibots. Praiseworthy had stumbled across the robots but expressed grief for his mother, causing the Emojibots to grip him in order to murder him. Steadfast shot one of the two, causing an angry response to which it summoned Vardy to kill the colonists. The Doctor realised that the Vardy were a lifeform, so wiped its memories of the events that had occurred when he found a reset button on an Emojibot. This meant that the Vardy thought Gliese 581 D belonged to them. The Doctor started a negotiation between the humans and the Vardy in order to try and allow the humans to live with them. (TV: Smile)

Minor references
While floating in Titan's low gravity, Bill mentioned "killer emoji robots". (COMIC: The Soul Garden)

Behind the scenes
The Emojibots were not named on screen in Smile, only in the credits.