Sadly your internal display is bad. You’ll need to replace it to fix this. Here’s the gunned you’ll need to follow: [guide|55925]
Now the shoe drops! These displays go for big bucks! This is because many had bad anti-defective coatings. A new one goes for over $600 !! A used one can be half of that if you can find one [https://www.thebookyard.com/advanced_search_result.php?keywords=+661-02532|15” Retina MacBook Pro Display Assembly, Apple P/N: 661-02532]