Quicktime: Grey background upon resume in fullscreen mode
This is an annoying bug in QuickTime Player. There isn't much you can do about it, I'm afraid.

Closing and then reopening the file usually fixes it, but if it doesn't you can try logging out and then back in.

If QuickTime gets so 'confused' that it can't open properly, quit using cmd-opt-q to prevent it from saving what's open. That should allow you to open it up and try again.

As always, file a bug report on the Apple website.