In any pond, algae is always present. It is natural and good to have algae in any organic water body. Keep in mind that algae is a plant that Mother Nature uses in the following way:
Here’s what happens
Your fish eat food, which they then turn to waste.
That waste is ammonia, which is converted biologically to nitrites.
Via the nitrogen cycle the nitrites are turned into nitrates (a fertilizer).
The nitrates feed the pond plants. Remember that algae is a plant, thus if there are no other plants in the pond, or a limited number of aquatic plants in the pond, the algae will grow to “pick up the slack”. – This may be why you have an algae boom; a green pond.