How do I get rid of mysterious stuff displayed on my cellular's screen?
You most likely switched on several feedback settings in Settings&rarr;Developer Options:


Developer Options (Source: AndroidAdvices.Com)

In these screenshots the reason for the "white dot" wherever you touch the screen is already marked. Additionally, you probably have enabled "Show CPU usage" as well (right screenshot, lower third). Just turn everything off here which you don't know what it's for; by default, nothing is checked in this menu.