Remove white mark

[Ofnuts]

Using rather plain tools:

- Using the Lasso, make a selection around the hazy area
- Select>Feather that. I used 30px on the provided image , adjust in proportion of your actual image size
- Color>Hue-Saturation and decrease Lightness a bit

   

- You can also add a moderate amount of sharpening (Filters>Enhance>Sharpen)