Paul wrote It seems to grow from the rocks from
which the water runs over.

There are lots of different kinds of algae.
The ones pond keepers deal with a

suspended algae - tiny cells that float in the water and make the water pea
soup green.
string algae - fibrous strings of algae that looks like long, green hair
substrate algae - looks like a fuzzy green sweater and grows on surfaces

Algae in waterfalls or water returns tends to be string algae or substrate
algae.
String algae - just remove.
Substrate algae - leave alone or scrub off if it really offends your eye.

All algae grows because it has the nutrients it needs. Sun, fish waste, rotting
plants, blown in dust and dirt, fertilized run off, fresh water.

Everywhere else there is no algae in the pond.

Oh, there's algae in there. You can't have water outside and not have algae.
There is algae in the waters of the artic! This is tough stuff. It is just not
in concentrations that you can see it. For now... One thing about ponding,
never rest on your laurels.