Hi,
1.Is there any changes you've made before getting this grey cube?
2.Kindly perform clean boot to verify if any third party process resulted in this grey area.
How to perform a clean boot in Windows
https://support.microsoft.com/en-ph/help/929135/how-to-perform-a-clean-boot-in-windows
3.Please try to unlock taskbar and move it to the right of the computer or other places and verify if the grey area will change accordingly to confirm whether or not it is physical problem related to monitor.
Looking forward to your update.
Thanks,
Jenny