Why is there a grey rectangular oval at the bottom of my screen?

[AC Question]

Every so often, a rectangular oval appears on my screen and on apps such as Snapchat, it deletes the current photo I'm about to post or while I'm texting, makes the conversation go blank (so I have to click the back button and swipe on the conversation again, sometimes losing the messages). On YouTube, it pauses the video and on an app called Showbox it pauses the video for a second or two but continues playing afterwards. It appears on all apps, it's appearing as I write this out. The problem is that it stops my phone functioning properly. I have a xiaomi mi note. Thanks for reading.

 

[Almeuit]

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