Monitor Problem - weird 'stain' that spreads

I have this weird monitor problem. My monitor is Dell U2311H, 6 years old. There is a stain inside the monitor that spreads. It is not on the surface of the monitor.

Here are photographs.

The stain does not show up on alt-printscreen screenshots.

It has been growing for the past week.

Is there any way to fix this at reasonable cost? Who can repair this, any suggestions? Or am I going to be forced to junk the monitor?

(Also willing to hear from anyone who will buy from me this monitor.)