How to implement 3D Minigame Like House Flipper Window Cleaning

Question

I'm developing a game in Unity 3D. The basis of the game is that it takes place in a room, and clicking on different objects launches different 2D minigames. I want to implement a floor-sweeping minigame, where you click on a broom, and it brings up a picture of a floor with dirt on it, that you use your mouse to "clean", like how you clean a window in House Flipper.

I'm having difficulty figuring out how to implement this in 3D. In the research I've done, I found ways to implement this in 2D using sprite masks, but this doesn't appear to be a feature in the Unity 3D UI elements. How would this have been implemented in House Flipper? And what's the simplest way to approach doing this in Unity 3D?

Any help or pointers to subjects to do more research on would be greatly appreciated. Thanks.