Its probably just dust and such settling on top of the water. This, in combination with protein buildup from under the water, can cause a pretty thick film. Like I said, cause some movement at the water surface and see if that takes care of it. I'll almost guarantee it will.
Water exhibits high surface tension, meaning it's hard to break apart water molecules at the surface (think of how water strider bugs can skim across the surface). They provide a level, almost solid area, where dust particles can easily settle. If the surface is big enough, you get lots of dust. Especially if you have a fan blowing, or the tank in front of an air conditioner, etc. More air = more dust.