This sounds like a question that a six year old would ask, but where exactly does algae come from? You can start with a sparkling clean pool, pond, birdbath, or aquarium and after a while there is algae. How does it get in there?
Where does algae come from?
It's a small plant micro-nutrient that grows as soon as the water conditions are right. What size pond do you have? A UV filter will kill the algae and keep it crystal clear.
It's just a doggie pool - 3 ft across & 8 inches deep - but the frogs and wildlife love it. I had horsetail rush and dwarf papyrus, but had to remove them because they were taking over. I don't dare put anything requiring electrical wiring for the safety of dogs and men with lawn mowers. I need to replace the solar fountain that failed.
You can order pond bacteria (or get it for a small pond that size from Lowes or a Pet Store like PetSmart, etc) Good bacteria is what balances the water and keeps it clear. If you're getting frogs, the tadpoles eat all decaying debri that adds nutrients to the water and becomes food for algae. In a pond that size, the tadpole/healthy bacteria combo will keep your water very clear of free-floating algae (green water kind) Algae along the sides will always be there, although the tadpoles will munch on it.
Also more surface coverage ie: pond lily, duck weed...will discourage the algae bloom.
My question is where does the duck weed come from? All you have to do is put out a saucer of water and soon its full of duck weed.
Duck weed could be from wind born seeds. If it's what I'm thinking of it's very small leaves. We get one like that here but I never bring it home to my ponds cause it clogs the skimmers and then goes down into pumps and wrecks them. Don't ask how I learned that.