Super Mario Run is currently running a huge in-game event for the next couple of weeks.
https://www.youtube.com/watch?v=Qpdfg5km60w
Super Mario Run launched to great fanfare, but content updates have been oddly absent. In its stead, Nintendo is introducing events such as the current one that is being run. Between 1 February 2017 and 20 February 2017, Super Mario Run is running the Golden Goomba event which introduces these special monsters to the World Tour mode. Golden Goombas will appear throughout the courses and defeating them will net you 30 gold coins.
In addition, you will be also be able to collect stamps stamps for every Golden Goombas defeated, where a completed card will make a new building available in Kingdom Builder mode.
Source: Gamespot.