Red square icon on top right of screen?
What does a red square in the upper right corner of my iPhone mean? It’s not a circle but a dark red square? My microphone accessibility is turned off!
iPhone 6s Plus
If it appears when you are on a phone call that is expected, as you can’t make phone calls without a working microphone. Depending on display settings it may be red and may be a square→About the orange and green indicators in your iPhone status bar - Apple Support
If you are not using a microphone the most likely explanation is that it is part of your wallpaper image.