Pond-surface film, also known as SCUM, is a thin layer of organic material that forms on the surface of a pond. It is composed of algae, bacteria, and other microorganisms, and can be seen as a thin, greenish-brown film. SCUM can cause problems in ponds, such as reducing oxygen levels and blocking sunlight from reaching deeper water. It can also lead to an increase in algae growth, which can lead to further problems.