what is the small shadowy puzzle piece at the bottom of the screen

[AC Question]

I have a small (about a quarter inch) see through square that looks like a puzzle piece on the bottom right corner of my Samsung S4 screen. It just showed up about 4 days ago. I do have a puzzle app, but I've had it for several years and have never seen this before. Anyone else have this issue?

 

[B. Diddy]

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