Wow that is really cool. Do not worry about the green stuff. It is purifying the water and is a good snack for the fish, if they can get to it. it is important to know the levels of ammonia, nitrite, and nitrate in your pond. If you dont have one already, I would suggest buying a test kit that uses drops, not strips. Ammonia and nitrite and very toxic to fish but nitrate is not toxic until it builds to higher levels. One topic to search would be "tank cycle" or "pond cycle". It will help you understand how fish produce waste by breathing and excreting waste and how it becomes ammonia that is processed by bacteria in the water that turn that into nitrite and then nitrate. If you take care of the water, the water will take care of the fish.