Your neutral line (white) also acts a ground wire. The extra green ground wire is in the event a wire comes loose in a metal electric box the electricity will still be grounded to your house and not electrocute someone or cause a fire. As Sphelps says as long as you are grounding from your ballast to your electrical outlet you will be fine. You don't really need to add another ground from your wooden canopy to the house as the neutral wire is still grounding your fixture and the wooden canopy won't allow electricity to conduct through it in the event of a loose wire in the canopy. Just unplug the ballast before you do any work in the canopy such as bulb changes and you should have no problems.
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