Rectangular box on-screen

After upgrading to Mountain Lion, a rectangular box started appearing on screen. The cursor is in the box, so when I move the cursor, the box moves with it. I've checked out System Preferences, but there are no settings to turn off. I ran a utility program to diagnose, but everything was normal. Upgraded to Mavericks and the box is still there. How do I get rid of it? See the screen capture below and note the rectangle around the Apple logo.