That is WAY to much headspace! Once fermentation slows, it's important to either rack to a smaller vessel, or otherwise top up to the top, to avoid oxygen contact. Mold is one problem, but so is bacterial contamination (like vinegar-causing bacteria) due to the growth they have in an oxygen rich environment. Also, oxidation is a huge risk.
I'd rack to a smaller vessel today, and hit it with 1 crushed campden tablet per gallon as well to try to save it!
Broken Leg Brewery
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