Emojibot

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

The robots, 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 the Vardies were able to keep track of what their emotional status was. When the pair found out that the colonists were murdered, the robots read that they were upset so began chasing them and surrounding them in order to execute them. The Doctor found out that by smiling and pretending to be happy would trick the Vardy.

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 Vardy. Praiseworthy had stumbled across the robots but expressed grief for his mother, causing them to grip him in preparation for murder. Steadfast shot one of the two robots, causing an angry response to which it summoned Vardy to kill the colonists. The Doctor realised that the Vardy were a new intelligent lifeform, and used one of the robots to wipe the memories of the consciousness. 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) She also mentioned them to be "Weird robots" (TV: Knock Knock)

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