2017 iMac shows random picture square in my desktop background.

I recently was using Canva AI to remove background from images and it was putting a random background behind my pictured object. Now, at a variety of odd times, i have noticed a square of a different picture on my desktop background. (yes, I have completely quit Canva) If I select/cursor over it, the small square goes back to what was supposed to be in my desktop picture. Anyone else have this random weirdness? When it does it again, I'll try to screen capture what it's doing.