Q: Iphone 5 Screen Halo

Hi everyone,

I've owned an iPhone 5 for over two years now. Over the last few weeks I've noticed a greenish halo around the outside edge of the screen (See image below). In the image, the halo is especially noticeable around the bottom right corner of the iPhone screen. Can someone tell me what the cause could be, if my only option is to replace the screen, and how much money I may have to spend for replacement?

Thanks for you help!