Generalized, the term I see and hear the most for these kinds of things is "floating" text. It can be more specific: Dragonrage's answer mentions combat text or damage text, but it isn't restricted to a combat scenario. Floating text is often used to contextualize numbers or short phrases by causing it to float near whatever it's meant to represent - damage floats off of or pops off of the damaged creature or character (or their hp bar); gained resources appear as +1's near your stock of that resource, the places you collected or delivered it, or both; a reminder on which button digs for treasure could rise from the earth as your character nears the big X on the ground.
Sometimes I think the floating text that "pops" up and then falls away should be called something else, since it doesn't exactly "float", but typically I still hear it called floating text, or it's replaced with a more specialized term like "damage indicator" or "pop-up reminder".