why the algae?

I have a small pond, about 100 gallons (its really on old corner bath) It is
well covered with floating plants, also some that grow from the bottom like
lillies but they arent, lots of oxygenators, marginals, a fast running pump
for the size of the pond, with a fountain and a spitter. The pond is near
the house and well shaded, with very little sun. The water is clear and the
fish healthy, and they eat everything as soon as it touches the water
surface. So why, I wonder, am I always pulling out yards of string algae?
Any ideas?
Barbara