All the rocks in our pondless waterfall were covered with brown “gunk”. Shut it down for a week, brown stuff dried up like dead leaves. Power washed everything, started is back up, looked beautiful. Three weeks later the brown “gunk” is already very noticeable. What is this stuff and is there something I can use to prevent it?

This is a form of algae. Next time when it dries off, remove it instead of washing it into the basin. Everything that you flush into the basin will have to decompose and the algae will come back. You can use an algaecide 2 times a month. We recommend Microbe-Lift Algaway 5.4. You can purchase it on our website. Comes in various sizes.