You need to do a series of water changes and add more plants. The algae is showing up to feed on nutrients in the pond. If you change water out, you remove those nutrients and the algae at the same time. The fish will continue to produce nutrients as well as any leaves or organic debris that falls in the pond. Green plants will compete with the algae for those nutrients.
In the long run, you can utilize barley straw extract to help prevent/eliminate green water. However, nothing does a better job at preventing green water than a good bio-filter, low nutrient levels, and green plants. The more plants, the better.