First of all - apologies, I know it's probably fine. I'm not a big believer in infections, but I'd still like to know what this is.
Second - due to a lack of equipment I had to swap out the airlock on one cider (#1, finished fermenting, clearing) to another (#2, fermenting now) for a night. I plugged #1 up with kitchen roll instead and got to the HBS as soon as possible.
Upon returning, the cider had a film on the top of it that sort of dissolved in as I shook it. I've attached a sanitized airlock back on and again, I'm waiting for it to clear. Ingredients;
Montrachet Wine Yeast
Wine & Beer Finings