There is no canonical name as far as I'm aware. So, instead I will provide terms that I hope will be useful for you.
And while I cannot provide a definitive source these terms, I do not claim to have come up with them, instead that I have picked them from assets, tools, and other developers.
So, feel free to take my answer as hearsay. In fact, I invite you to search the terms and decide if you will use them for yourself.
In that order of ideas, I shall point out that "Dialogue Marker" has other meanings aside from the one I give below, including the icon used to indicate that you must press a button to advance a dialogue.
Also, if you search the terms "Quest Marker" will give you mostly icons and models, similarly "Status Marker" will yield status tokens for tabletop games, but for the other look for tools that use them, since they don't yield many visual results.
Overhead Marker vs Quest Marker
The most general term for some visual element floating above the head of a character is Overhead Icon, although I prefer Overhead Marker because the other related terms are also called Markers.
However, even more popular is the term is Quest Marker, but they are not limited to be over the head of characters (they might mark places or items too, and might appear in maps or other navigation aids). Also you might want to place other things over the head of characters that are unrelated to quests.
So there is overlap between Overhead Marker and Quest Marker.
Other kinds of Markers
Aside from Quest Marker, these are the related terms that I can find:
- Overhead Marker: An icon floating above the head of a character. In general.
- Status Marker: visual elements either floating above the head of a character or below its feet, which tells you something about the current status of said character. These also include nameplates (floating text), health bars, status effect icons, and active selection indicators. And yes, I'm counting The Sim's Plumbob as an example.
- Dialogue Marker: An icon floating above the head of a character, that tells you that the character has something to say. A common example is an speech balloon icon. Sometimes an information icon (an "i") or interrogation if they give you instructions about an active quest. And some games also have a dedicated icon for shops. Plus the popular exclamation mark for quest givers (which is a Quest Marker too).
- Interaction Marker: An icon floating above the head of a character or above an item to indicate that an special interaction is available.
- Waypoint Marker: An icon that might appear in maps or other navigation aids, and also on the terrain, to indicate a location for the player to reach. *Usually an arrow or map pin icon.
So a particular Dialogue Marker, Interaction Marker, or Waypoint Marker, could also be Quest Marker if it is indicating something that is part of a quest.