How to I remove this transparent grey box left mid in picture on iPad.

That looks like you've been using picture-in-picture, that lets you view a minimised video from one app whilst doing something else. What happens when you tap the left-most of the three icons on it, does that not maximise it so that you can see which app it is ? Or can you tell which it is via the iPad's multitasking bar ?

All apps ? And for Safari, after closing it, you've cleared its cache and history via Settings > Safari ?

Have you closed apps via the iPad's taskbar ?

iPad Pro 2nd Gen same issue started on iOS 12. Extremely frustrating bug. At this point hopefully it will be fixed in iOS 12.1.