Algae is a naturally occuring problem particularly in the summer. Algae is caused by a number of factors, and usually by a combination of them.
Too much sunlight falling in the water's surface is the most common issue. Use floating plants like water hyacinths and water lettuce to shade the water. 60-70% coverage is ideal
Warm Water: If the water is shallow, it can heat up and create an algae bloom. Make sure your pond is deep to allow it to cool down. too few floating plants also enable it to heat up quickly.
Too many Nutrients: Over feeding of fish or plants, or excess nitrogen create an environment that algae loves. Increase the ratio of plants to water as plants will feed on the nutrients denying it to the algae.
Too many Fish: Fish excrement becomes nitrogen (nitrates) that become nutrients for algae. Reduce the number of fish or perhaps consider a better filtration system in your pond.
Standing Water: Water that is stagnant and non-moving can warm up and contribute to an algae bloom. Consider adding a bubbler, fountain or a pump to help circulate the water gently around your pond or water feature.