Pond scum is a term used to describe the slimy green or brownish layer of ALGAE that can form on the surface of a pond. The two crossword solutions for this term are ALGA, which is a four letter word, and ALGAE, which is a five letter word. ALGAE are a type of aquatic plant that can be found in both freshwater and saltwater environments. They are an important part of the food chain and provide oxygen to the water.