Ponds and water features that are exposed to sunlight can turn green and murky very quickly. The two most common types of algae are “green water algae” (aka pea soup algae) and "string algae” (aka hair algae or blanket weed). These unsightly algae can be easily eliminated by using algaecides. Be careful as some can harm plants. API algaefix is safe to use with live plants and fish.
Green water algae are tiny single celled organisms that can turn your tank into pea soup. The algae is so tiny they pass through a pond filter. Use a chemical algaecide to destroy the algae, and then use a flocculant (like API Ecofix) to clump the dead algae together so the pond filter can break it down. As your filter traps the dead algae remember to wash your pond filter out regularly over the next few weeks so the filter can continue to work effectively to clean the pond.
To keep your pond clear you need to establish an ecological balance. Do this by providing your pond with good water circulation and some floating or submerged plants. They compete with algae for light and absorb excess nutrients in the water so the algae does not have a chance to get out of control. For koi ponds or ponds exposed to alot of direct sunlight, a good pond filter and UV clarifier may be necessary to keep your pond crystal clear all year round.