Why is there a black shadow in shape of network icon on my screen?

There is a black shadow on top left corner of screen that’s more visible on colored screens like red blue and not very visible on white or really light screens.

It started out a small triangle with two sides parallel to the top and left side of phone and 2 mm on top and 3 mm on left about 4 days ago. Today it has changed to be in the exactly like network icon - four rectangular bars at the same place.

I can’t take it to the Apple Store right now due to quarantine and can operate the phone just fine right now. I’m worried that it will grow though.

I haven’t dropped my phone or gotten it wet or overheated or anything. I’m extremely careful in taking care of it so I’m unsure how this suddenly happened.

Edit: Added picture but the camera doesn’t have a good resolution so it might not be clear. The issue is at the top left corner.