Here is a common growth pattern. Some green hair is on the strings, and some dark/black slime is on the rocks. Although there are some differences between the materials of the rocks and the strings, the main reason for the difference in growth is probably the proximity to the lights: More light is in the middle, and more light usually grows lighter/green color growth unless the nutrients in the water are too low:
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Inventor of the easy-to-DIY upflow scrubber, and also the waterfall scrubber that everyone loves to build:
http://www.reefcentral.com/forums/showthread.php?t=1424843
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