Factors that can contribute to algae developing in your pond include excess nutrient levels (possibly due to an overstocked pond), high water temperatures, an insufficient filtration system, too much sunlight, overfeeding, and high pH.
If you experience an algal bloom in your fish pond the first thing to do is a partial water change. This is only a temporary solution and you will still need to act on the cause of the algal bloom. Clean the pond, remove any decomposing biological matter from the bottom. Prevent water run off from lawns entering the pond water.
Chemically controlling the algae should be used only as a last resort when other methods have failed.
An Ultra Violet Clarifier (UVC) is a safe method for killing algae and other germs.
Adding aquatic plants will help reduce excess nutrients in the pond water.
Install a large biological filter to keep nutrients at a minimum.
The difference a UV Clarifier can make to pond clarity.
The video below shows ponds with and without a UV Clarifier and shows the big difference they can make to pond water clarity.
For international viewers the best prices on UV clarifiers can be found on Amazon (see below).OASE Bitron 55C UV Clarifier