Rectangle box on iPhone screen

Hi,

I am having iPhone 4S with iOS 9.3.1. Suddenly, The touch is not working and It was hanged. So, I restarted the phone by pressing the Home and wake up button at the same time. After the restart, On top of a button, there is a black bordered rounded rectangle button. It accepts touch at the second time. I get rid of this issue. I was not able to get resolve this. I have attached the screenshot for reference to this issue. From that screenshot, you can see that, there is a black rectangle box on the Back button of the General Settings page of Settings app.

Please help me, This is much irritating.