Grey box on mac screen
I have been getting a grey box appear on the screen and can only get rid of it by restarting the macbook air. It's the box obscuring the word "question" on the picture below. I noticed today that it had appeared on one of the other imacs we have where I work as well. Any ideas of a quicker, less annoying way of getting rid of it than a restart? Cheers.
MacBook Air (13-inch, Early 2014), OS X Yosemite (10.10.4)