Scum, an oily film, or yellow-green dust on the surface of a lake makes it look contaminated but, in general, nothing is wrong. In fact, something natural is probably occurring. An oily film in midsummer is most likely caused by insect cases that become concentrated along the shore by wind after a hatch. Insects can hatch any time from early spring into September. As the cases decompose, they sometimes give off an oily film. Yellow-green dust floating on the surface in late spring and early summer is pollen from nearby pine trees. In contrast, an algal bloom is green to blue-green, might look soupy, and can form thick, slimy looking surface scum.