Is this outlet wired to a switch correctly?
As long as you ran a ground wire from the receptacle to the box, you should be fine providing the box is grounded.  It's not uncommon for older systems with metal boxes to have the box grounds connected.  Make sure they aren't just bridging neutral to ground anywhere to fool the tester. 

You do need to run ground wires to the box on receptacles. A receptacle can also ground through direct contact between metal yoke and box, as you have with that switch; but not if the box is dirty or rusty (as your switch is) or if it has the little paper squares to capture the screws (as your switch does). 

The rules are easier on grounding switches; your switches can ground through their screws despite the material preventing the yoke from hard contact.