This RAIN2 had light running on one side only. It shows a few things:
The non-light side has a dark area at the top; this is where light is having trouble reaching the "roots" and thus grows dark/black slime, which does not hold on well. Having light on the other side would have helped prevent this.
The lit side has growth which has fallen onto the tray; if light had been on the other side, this growth could have held on longer. Growth on the tray does not continue to grow much because of lack of flow and light, unlike the RAIN-8 which is designed to continue to grow on the tray.
The fallen growth, although not growing much on the tray, is still very easy to clean due to the removable dome and tray. This is the opposite of box style scrubbers where you have to reach down into the box to scrape the bottom, that is, if they ever grow this much at all because their lighting is usually much weaker.
The pipe slot should probably have been brushed clean on each cleaning; it looks like it might have not been brushed in a while, and thus some flow is being blocked. That's the reason for a black pipe: it reduces light getting into the slot and growing growth which blocks the flow.
This person has put a piece of material on the tray to reduce waterfall sounds. However if you just control the flow so it's consistent across the bottom of the screen, there should be almost no sound. And if you lower the tray down to the waterline, it will be silent.