What is this random graphic that showed up on my screen?

[Microsoft Windows]

This 1" x .25" black rectangle with a red square and part of a white letter appeared on my Chrome screen a few minutes after I signed onto the internet tonight. I'm in a public parking lot. I signed into Optimum Online, a free public wifi. A few mins later, THIS.Can't figure out how to get rid of it. Switched internets (dis-connected the Optimum, signed on via Dunkin' Donuts and it still won't go away). It's also on the screen when I'm on Word. What is this thing plz and how do I get rid of it?

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