There are two possibilities here:
- Dust cause in between the cover glass and the LCD
- Something was resting on your keyboard when you tried closing it or something was placed on top of the lid and squashed the LCD enough to push away the liquid between the glass plates.
With the display off (black) shine a light at an angle to the display do you see the white area? If you do it's dust. Which you can clean by carefully taking the cover glass off. If not the display is damaged and it will need to be replaced. In either case you may want to leave it alone and just live with it if it's minor.