I have a black box on my iMac :(

I am having a lot of trouble with my iMac recently, as it has had a lot of problems. However i have fixed most of them problems by upgrading all my apps and software. The most annoying problem has sadly stayed and its very annoying!

A little black box appears on whatever i click on and stops me from scrolling down on any web browser or any website.

What should i do ?

I hope someone can help me😟