Thanks, OC, - I guess I though the algae was somehow growing on a screen in a container outside the main tank. I haven't followed that process closely.
Yes, it is normally outside the main tank, but water is being pulled from the main tank to the scrubber, and the flow returned to the main tank. It can also be done with the HOB filters.
Basically, you are flowing the tank's water over the screen. The added light gives the algae the ability to grow quickly. The more light, the more CO2 and other nutrients the growing algae will take in. The water being returned to the main tank will have less nutrients in it (so it 'starves' any plant or algae IN the main tank), and more oxygen.
This has been used on saltwater tanks for decades (built into the 'sump' under the main display tank), and is just now getting a foothold in the freshwater realm.
More oxygen is good for fish. Fast growing plants in the main tank can produce more oxygen though. If you see a well-done planted tank, you will often see bubbles streaming from the plant leaves/stems (this is referred to as 'pearling') when the lights are on. This means that they are producing so much oxygen that the water cannot dissolve anymore. The oxygen bubbles float to the surface and are off-gassed.