Pond Problem is a term used to describe the accumulation of ALGAE, SCUM, and other organic matter in a pond or lake. ALGAE is a type of aquatic plant that can grow in large numbers, forming a thick layer on the surface of the water. SCUM is a slimy, greenish-brown substance that forms when organic matter decomposes in water. ALGA is a type of single-celled organism that can form colonies in water. All three of these substances can cause problems for ponds and lakes, such as reducing oxygen levels, blocking sunlight, and creating an unpleasant odor.