There looks to be a lot of headspace in those jugs. Once fermentation slows down, after about 5-7 days in primary, you want to make sure it's always topped up to avoid any chance of mold or bacteria taking hold plus to avoid oxygenation. A darkening color could just be the wine clearing, or it could be the beginning of oxidation.
I can't tell from the picture, unfortunately. But "white scum" sounds really bad. It probably came from allowing that much headspace. I'd definitely rack it to something smaller (wine bottle, growler, etc), treat with a campden tablet, and top up and airlock. I probably wouldn't combine it, as it may just ruin the other wine as well.
Broken Leg Brewery
Giving beer a leg to stand on since 2006