Foggy screen after some using

Hi guys,

I have a problem with my girlfriend's iP 5C. After some time of using Facebook, Safari etc apps screen goes very bright and "foggy". Also some dark spots appears and I can see shadows of icons on the background. The problem occurs once a day but also it disappears randomly - either when she continues browsing web or she leaves phone locked on the couch. Phone was bought second-hand and I am not sure if display is original. There are photos in link below. Do you have any idea what it might me?

http://imgur.com/a/N55n0