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!
Yes too much head space but it don't look like mold or anything out of the normal. Every lemon mix I have fermented had the white stuff on the sides and on the bottom as well. Rack off and top up no problem.