I don't know that you need to do anything about it anyway since you have a free flowing situation rather than recirculating like most of us. However, I am one of several here who don't worry about string algae. It seems to really like flowing water and I have a stream as well as pond. I occasionally grab some of it out along with a lot of the crud that has been filtered into it. It is really my filter.

Waterbug_guy (who is one of our sometimes prolific members) has done extensive observations that indicate that string algae competes against single cell algae types that create the 'pea soup' problems. I certainly have never had pea soup problems and I am happy.