3D growth is what you get when you enclose the scrubber in a box that is small enough such that the growth touches the walls and traps water like this:
I believe it was SM that first noticed this effect in his SM 100 scrubber, the box would fill up with algae growth and once this started to happen, the growth seemed to take off somewhat exponentially for a period of time.
I've had some harvests where the growth was so densely packed, it came off like a slab.