The best way to treat algae is to deprive it of what it needs: nutrients and light. Floating plants (duckweed, water lettuce) or floating-leaved plants (water lilies, lotus) will severely cut down the amount of light that reaches the algae. A good filter, along with plenty of plants, will help pull nutrients from the water; making sure organic debris doesn't accumulate too much also helps.
Algicides are short-term treatments at best, and many of them are lethal to plants and invertebrates and can also be toxic to vertebrates. However, a UV sterilizer can severely reduce the numbers of planktonic algae and photosynthetic protists, the causes of green water, without harming the more desirable pond inhabitants..