Which plant is commonly referred to as "pond scum" and is recognized by its greenish brown mats formed on the water's surface?

The plant commonly referred to as "pond scum" is filamentous algae. This term generally describes a variety of algae that can form dense mats on the water's surface, often appearing greenish-brown. Filamentous algae thrive in stagnant or slow-moving waters where they can multiply rapidly, especially in nutrient-rich environments. These algae can affect water quality and aquatic ecosystems by limiting light penetration and altering the habitat for other organisms.

Sago pondweed, curly-leaf pondweed, and wild celery, while they may also be found in aquatic environments, do not exhibit the same mat-forming characteristics as filamentous algae. Sago pondweed and wild celery are submerged aquatic plants that tend to grow in structures rather than forming surface mats, while curly-leaf pondweed is a submerged species that can become a problem in certain situations but does not resemble "pond scum." Thus, filamentous algae is the most fitting answer due to its defining characteristics and common perception as "pond scum."