Hard to tell. It sounds like the results of summer plankton bloom cycle. That( gooey film ) was described to me by a fisheries biologist as the cytoplasm from dead plankton (died overnight)with no light. During the day the live plankton moves to the surface for light adds O2 through respiration ,reduces visibility and dies during the night leaving the cytoplasm skim.

Maybe Bill can give us a better explanation.