Apple iOS 11 is chock full of problems and bugs—here’s another one, and see the app download bug. I think iOS 11 was rushed out the door by calendar schedule (as usual) foisting manure-grade software on hapless customers. See for example Surprise! Apple Releases iOS 11.2 to Combat Reset Loop Bug.
As shown below, the entire screen is blurred. This is app specific, for example, the camera or Contacts or whatever. When it happens, that app become unusable and the only solution that I’ve found to restore functionality is to power down the phone and reboot it.
NewerTech cases have saved my iPhone 6s Plus and iPhone 7s Plus many times from damage.