The grey looking wires are the white ones, which go on the silver screw side. The black is the ones that go on to the gold screw side. They are "clumped" together, because they wire nutted them together and that is fine. You really don't need to mess with putting a green wire to the box if you don't want to, because if you look at the new plug itself, you'll see the green crew on the bottom is touching the rest of the silver where the top and bottom screws are to put them to the metal box. It does the same thing. If you want to, you can put wire from the green screw to the plug, and use a self drilling screw to the box itself also. That's all it takes.
