Macbook Display Screen Coating
This is a known issue for Retina MacBooks and Apple is aware of this.
You can search for 'staingate' for more info.
There is a replacement program going on, you can read about that at MacRumors website.
You'll have to make an appointment at a genius bar where they will check if it is still covered.

Apple will replace Retina displays on affected MacBook or MacBook Pro models for free within three years from the date of original purchase, or one year from October 16, 2015, whichever is longer. Affected customers that have already incurred out-of-warranty costs may be eligible for a refund through AppleCare support.
Affected customers can book an appointment with a Genius Bar or visit an Apple Authorized Service Provider to determine if their MacBook is eligible for coverage. Apple does not currently plan on announcing the Quality Program publicly on its support website, according to sources, but may contact some customers directly.