maheeni Said:
"It is residue, looks like a backlight issue."
-------
Troubleshooting Backlight Issues on a MBA:
This may be a ghost image. So, try the following...
A. Links to Use:
B. Use an External Display:
Plug in an external monitor to your Mac, and see if you get thi issue. If not, then is is an issue with the Mac. If so, then is is an issue with the entire macOS itself. If all there are no USB-C ports, then use an adapter: Belkin USB-C to HDMI Adapter - Apple
C. Boot in Safe Mode: [Hold Down: shift key, upon boot], In Safe Mode, Login Items do not run at login, certain preferences are set aside, and damaged system files are then reset. So, once booted in to Safe Mode, wait 30 seconds, and then boot up normally, attempting another update install.
D. Get this Diagnosed by Apple:
Apple might fix it free of charge, if found to be be faulty device. Set up a Genius Bar Appointment at your local Apple Store. There, it will be looked at and you'll be provided with your options.
Contact Apple Support: