The algae is growing because the nutrients are available to support its growth. A small amount of algae is good for the aquatic garden since it absorbs excess nutrients in the water, helping to keep it pure. Uncontrolled algal growth depletes oxygen in the water and makes the water inhospitable for the fish.
Make sure that you have not overstocked your pond with fish. Also make sure that you are not overfeeding your fish. Uneaten food is source of nutrients that translates into algal growth, and a large population of fish produces a large amount of nutrients when they excrete waste products. A pond biofilter can help remove excess nutrients and keep the water clear. You can also add water to the pond periodically to dilute nutrients if you have an overflow system that can drain excess water out of the pond. Barley straw discourages the growth of certain types of algae. Pond supply firms sell barley straw products that can be submerged in your pond. Dyes are available that can be added to the water. The black material absorbs the sun's light energy and deprives the algae of the light it needs to carry out photosynthesis and survive. Dye products have the added advantages of protecting your fish from predators and concealing plumbing and pots in your pond.
Many aquatic plant gardeners are tempted to drain the pond, clean it, and start over with fresh water when confronted with algae. This is usually counterproductive since the excess nutrients that cause algae to grow build rapidly in the fresh water. Frequent water changes and cleaning can make for drastic changes in pond pH and are stressful for fish and other aquatic life. Only drain and clean your pond when a substantial layer of decaying organic matter has accumulated at the bottom.