There are many different forms of algae. There is the type which turns the water green, which is called a mono cellular algae or blanket weed which can either be the long stringy type which clogs onto everything or the spongy cotton wool type which floats all over the surface of the pond or lake. Duckweed is very common, click on this duckweed link for more on this type of algae. Whatever type of algae you have they all have the same cause which is over nutrition. The causes of over nutrition are many and varied, the usual culprits are fish or duck excrement, rotting leaves, dead weed, lawn fertilizer washed in from surrounding areas by rain or too much sun.
Natural ponds can be polluted via the water table from a source far beyond your own boundary, as can stream fed ponds. Likewise, one of the biggest culprits these days can be spray from farmers’ fields which can be carried for miles in the wind and deposited on unsuspecting ponds.
Aquaplancton stimulates the beneficial, aerobic bacteria in your pond which multiply and spread. They then consume any over nutrition, causing algae and blanket weed to die out naturally. Aquaplancton did get the highest score against blanket weed in the Gardening Which? Tests.