I had a similar situation last year with my first year pond. I think we all decided that, becuase my part of the country was rich in iron, ironore, etc., that some of the film may be from natural mineral 'leakage' inherent to the soil. I saw it more BELOW my dam than in the pond itself. But with rain and cooler weather, it disapated. Haven't seen it yet this year, but the really hot weather is upon us. My .02.
