The "film" I saw on the media was a uniform light tan in color, and more concentrated where there was higher water flow. There was nothing that was large enough to hold or see on your finger. It did not cling to itself like algae or mold would have and readily fell off the floss when wrung out. It looked like a strained, purified version of what you would find contained in a undergravel filter - so I always figured it was bacteria. If bacteria is too small to see-What else could the film be?