If you just opened your pond after having it closed off all year then their are some steps to take to make sure you don't have a algae bloom to begin with. This is going to take some work but is worth it in the long run. What you need to do is to take all the debris out from the settling leaves and other matter that creates that sludge on the bottom of the pond. Getting rid of pond algae starts here by taking out all the fish and putting in a container with a air stone. Then drain the pond completely and spray out with a hose with tap water. I then scrub the pond with a scrub brush to make sure to get all the debris from the bottom. After hosing it all out fill it up with tap water with a conditioner added to it to get rid of harmful things to them. Then add the koi back to the water when it reaches similar temperatures. Also adding Pond Cleaner Clear as Crystal will help keep away algae blooms or green water for up to 3 months per bag. All you do is slit the bag and set in the middle of the pond.
To cut back on this yearly maintenance you should cover your pond with a netting all year long, you can't see the net but it keeps leaves, birds, debris and other predator from reaching your water or fish. Don't forget to add lilies, or other pond plants to help in your battle for algae control in your pond. The netting can also act as a rest for a tarp or board to cut down on the sunlight if you need to reduce sunlight for algae related problems from it. Also at this time you can add blue tint to the pond water to help reflect sun rays that in return helps you get rid of your pond algae.
Doing the following will help kill those excess nutrients hiding in the sludge on the bottom of your pond and will make your pond season a much more enjoyable one.