Hmm. I am going to offer a hypothesis: The milk is steamed, so that there would be foam at the top to meet the underside of the lid is not surprising. Perhaps the natural oils in the coffee mixed with the fat of the steamed milk provided for a film. Some things that could contribute would be using different milk types (whole, soy, etc) that might be steamed at a non-optimal temperature (too hot?). Espresso shots found in the cappuccino shouldn't have that much variance in the amount of natural oil they contain, and espresso is not filtered, but it also contains less oil than say, a French Press would produce. What type of milk was it, and how reputable was the place you got it from? Forgot to mention, while it's probably harmless, it isn't presentable or normal.