Yes, I have green water, ...

A good UV sterlizer will take that right out*. Assuming you have no
real water quality issues, the problem would be solved.

,,,some string algae on waterfall spillway rock...

An enzyme digester, if that bothers you?

Pond is 2000 gals, 24 inches deep. I have a bio-filter in pond-- and a
veggie filter at the top of the waterfall.

Let's here about the size of your biofilter (which type?) and veggie filter,
as well as your flow rates.

And the veggie filter... it is lively?

C//

* p.s., I'm new at this, and have been doing a ginormous amount of reading;
book learning isn't to be mistaken for hard experience. Be that as it
may, the written word is: UV sterilizers zap free floating single cell algae.