Stain on iPad screen

The photo doesn’t provide sufficient context to establish the actual position of where the “stain” appears on the screen - however, I make an educated guess that this will be adjacent to the magnetic Pencil charging dock - this being located at the centre of the long-edge of the iPad.

Your iPad needs to be assessed by an Apple technician.

You would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad. Here you will be in the best possible place to explore your available options for repair or replacement of your iPad:

Genius Bar Reservation and Apple Support Options - Apple

Your screen has suffered thermal damage due to a fault - for which the only remedy is screen replacement.

As previously suggested, your best course of action is to visit the Genius Bar at an Apple Store or AASP. Given the nature of the fault, this might be in your best interest:

Genius Bar Reservation and Apple Support Options - Apple

Alternatively, you might consider Apple’s mail-in repair service:

iPad Repair & Service - Apple Support

Either way, your iPad will likely be exchanged for a renewed/reconditioned iPad of the same model and specification - and if within warranty, at no cost.

It’s not a stain - but is most likely an area of the backlight diffuser that has suffered heat damage from electronic components associated with magnetic/inductive Pencil charging dock.

Unless the underlying fault is resolved, removing the source of heating, the damage to the may continue to spread. Seeking remedy sooner rather than later is recommended, as the underlying fault may have wider consequences.