Migrating Yellow / Discolored Spot on screen

I Have a spot of discoloration on the LCD of my Mid-2009 MacBook Pro. It normally appears as a yellow spot on a white background, but if the screen is displaying a dark image, then it appears as a bright white spot.

It also migrates with gravity depending on how my computer has been stored, i.e. if I leave my computer overnight with the left side down, the spot will appear in the middle of the left side of the screen. And by the end of the day at work, it will migrate to the lower end of the screen.

It's never at the exact edge of the screen, always about 1.5-2 inches from the edge.

Any Idea what could be causing this? Is it likely intrinsic to the LCD assembly so that if I buy a replacement screen it will be fixed?

Thanks for any advice!