Algae is natural in your pond. And it is beneficial. But to a point. As noted by Kasco Marine, there are several basic kinds. Planktonic algae are essential, single-celled plant forms occurring worldwide. A healthy pond needs this form of algae as a food source. Filamentous algae is typically found at the surface of ponds in "greenish mats." This kind of algae has little if any value to your pond and looks scummy. The third major kind of algae is attached-erect algae. The fourth kind to be mentioned here is blue-green algae, probably the worst when it comes to pond scum.

Algae is beneficial to ponds, as it provides a food source; in fact, pond owners who desire to raise trophy bass sometimes fertilize their ponds to keep planktonic algae production high. But algae poses several problems, too. For one thing, too much of certain kinds of algae is plain ugly. For another, too much algae is unhealthy. Photosynthesis requires sunlight, and algae blocks it.