The dreaded “ring around the carboy” strikes again! Quickie answer – your instincts are correct. I would rack to clean containers and, since you’ve already added SO2, and since I doubt your malolactic (ML) fermentation will reignite as a result, go ahead and give it about 10–20 mg/L total SO2. ML bacteria are notoriously difficult to get going again once they’ve decided to stop fermenting due to ambient temperatures, climbing alcohol levels, high acidity, etc. They are what microbiologists call “fastidious” organisms — that means they’re picky eaters and are extremely sensitive to changes in their environment. Unfortunately, most strains are extremely sensitive to sulfur dioxide and just won’t survive or ferment when there’s more than 10-mg/L or so around.
Though through the small amount of details it’s hard for me to define exactly what you’re facing, I would guess you’ve got a spoilage microbe (or microbes) starting to take hold. If your carboys have evaporated down a little bit, the ring can become more visually apparent. Often, when there is headspace in our wine aging containers (even if we are gassing the headspace) there can be a nice little environment (some oxygen, “food” like ethanol or acetic acid or residual malic acid in your case) for bacteria and sometimes film yeast to grow. Since you say that the taste and odor seem to be fine for now, I’ll advise you to simply nip any nascent microbial ingress in the bud and move along with the aging process in completely clean, topped containers.
As with any wine that has residual malic acid (meaning an incomplete ML fermentation), keep your containers topped up, keep your SO2 levels 25–30 (even higher if you have very low acid/high pH) and try to filter before you bottle. You do run the risk of a malic acid re-ferment in the bottle, but usually I advise people to just deal with the excess malic acid rather than throwing time and microbes at an MLF re-ferment that probably won’t work.
