This happens when you place neutral building in the game.
Now you can get rid of this icon - there are a few ways, depending on what you want:
a) If you don't want to show the house icon until you find the building with any of your units, change "Stats - Neutral Building - Shows minimap icon" to "false"
b) If you don't want the icon to be showed at all, you can change "Stats - Hide Minimap Display" to "true" - this, however, will completely disable its minimap display, meaning it won't be shown as a colored dot on the minimap like other buildings are.
c) Remove "Neutral" unit classification - units with this won't be shown in fog of war until you find them (neutral buildings are shown immediately in fog of war, but if you remove this classification, you won't see the building until you find it for the first time)
d) You could replace the minimap house icon with a blank icon.