Too much light, Most algae thrives on nitrate and phosphate, Your pond is over stocked, Not enough water changes, Medications in the water unbalances the stability of the pond and kills beneficial bacteria, New pond setup, Not enough filtration or a mix of any of these problems.
Some algae is good on the sides and bottom of your pond is great for koi & goldfish they enjoy eating it and its good for them. Excess however is not good, when you treat the water make sure you put aeration in the pond, dying algae takes oxygen out of the water.
How to fix algae blooms in your pond
Add 5ml per 300ltrs of water once, if your pond water hasn't cleared in 7 days, you can dose again. No need to look for a measurement vial it comes with one.
Windsor fish hatchery has been selling this product for the last 30years and has found it to be very successful in getting rid of most types of algae and green water, whilst still being safe for all your pond inhabitants, including fish,plants,frogs etc. Algae killer will keep the pond free of algae through out the summer months. Takes around a week to clear the green water in most ponds, for very thick algae may require a second dose.