Brown on Pond Waterfall and Surrounding Rocks

I've searched through this issue, but haven't found my exact problem
anywhere.

500 gallon pond with waterfall.
Crystal clear water (Maybe a slight brown tinge, but can see to 36"
bottom easily)
2000 gallon pressurized Cal Pond filter
1500 gallon Cal Pond pump
18V UV
Brown gunk growing all over waterfall and wall under waterfall

Because my waterfall is a little too powerful, I have to add appx 2
inches of carbon canned water daily due to the splash outside of the
pond.

What is the brown gunk? Dead Algae?
What affect will it have on my pond?