yucky green stringy moss

This sounds like string algae.
Most of us remove it by hand.
Some rec.ponders have had luck with
a product called 'string algae buster'

Algae, all forms of it, feed on sun, new water, fish waste, fertilizer,
fertilized run off, rotting plants, blown in dirt,
Increased waterflow can help break down the organic substances that can only
improve the health of your pond.

Good luck!
kathy :-)
A HREF="http://www.onceuponapond.com/"Once upon a pond/A