If you want your pond to stay clear of algae there are some good preventative measures to take. These include removing sediment from the bottom of your pond which is nutrient rich and can encourage algae growth. You can also reduce the amount of sludge build up at the bottom of your pond by cutting back foliage around your pond. Adding an air pump will you also increase oxygen to your pond which encourages beneficial bacteria that keep ponds cleaner. A stagnant pond without water movement is much more susceptible to algae growth.
However you may also need to get rid of existing algae. When you do this you must also make sure that you are not removing all the beneficial bacteria with it. These bacteria help to naturally keep your pond clean and without them the likelihood of algae growth reoccurring is high. Some additives will clear your pond of algae in the short term but in the long-term they will make your pond much more susceptible to reoccurring algae growth. For example the addition of copper sulphate does clear algae from your pond but will also kill the good bacteria which can take a long time to reestablish itself. To keep your pond balanced and healthy use an additive such as Microbe-Lift PL which considers the long-term repercussions to your pond and also install an air pump which keeps your pond aerated and less susceptible to algae growth.