Algae naturally occurs in pond water. There are multiple factors that contribute to algae form, like the water nutrient levels; exposure to light; temperature,; turbidity; and the general conditions found in a body of water all contribute.
Here are some quick tips to reduce the amount of algae in your pond:
If excess greening occurs, adding more Oxygenating plants will reduce algae growth to an acceptable level.
If the Algae becomes stringy (blanket weed), it may be that the pond water is alkaline. Check the PH of the water (a PH tester can be purchased from any aquarium) it should be in between 6-8.
Remove the stringy algae by hand or by twisting a stick through and removing as much as possible.