How do I get rid of this black semi-transparent rectangle covering my screen?

[AC Question]

I have an S5. An icon appeared (I probably accidentally touched something) and when I tapped this icon, a black semi-transparent rectangle appeared on most of the screen. This rectangle seems to block me from using the portion of the screen it covers. I can only interact with the top 100 pixels or so. I tried pulling the battery out and restarting, but that didn't work. Any ideas? Thanks!