I am attempting to replace an outside porch light. After removing the old light, I found the metal electrial box had the ground supply attached to a screw. The new light has a green ground wire and I was wondering if attaching it to the green screw on the bracket would be sufficient enough for grounding, or, do I need to remove the ground supply from the screw and pigtail it to the ground wire from the new light fixture? See the attached photo of the electrical box.
You're going to want to connect the fixture grounding conductor directly to the supply grounding conductor, using either a twist-on wire connector or crimp connector. You'll also want to use a pigtail, to connect the supply grounding conductor to the grounding screw on the metal box.
You should not rely on the support straps attachment means to provide an adequate grounding path.
The Grounding Wire can be seen in the background of the image, (you can see it has been painted white), generally, by attaching the mounting bracket to the housing, you create an earth connection, this is not the case, when the casing is made of plastic or has a non-conductive coating.
You can connect your earth to the green screw provided that there is a good earth connection, this can be tested relatively easily by using a multi-meter with a continuity test setting
Based on the image above, you can check that you have a good earth connection by connecting one lead to the earth wire(coming from the electrical outlet) and the other to the point which you will be using for an earth.