This is essentially uneven color fading in the film dyes, causing a localized color cast rather than a simple stain. In your example, blue and green have faded more than red, so the affected corner shifts orange.

A practical fix is to correct the faded area separately with a soft, gradual mask:

1. Make a feathered or gradient selection over the affected corner/edge.
2. Apply tonal and color correction only inside that masked area.
3. Use tools like Curves, Levels, Auto Color, or Color Balance to add back the missing blue/green and adjust brightness.
4. Compare against the less-faded part of the slide, then refine the mask so the transition stays smooth.

Examples from the community used:

• a lasso selection with heavy feathering, then Auto Color and a slight Levels darkening
• a quick-mask gradient selection, then Curves to set a black point and darken

So the key search terms are things like local color cast correction, faded slide restoration, and masked/gradient color correction. If you have many slides with similar damage, build a repeatable workflow using masks plus curves/levels rather than trying to remove it as a stain.