I get the same stuff in my low tech 20H. It's quite heavily planted and stocked with 5 cory cats and a BN pleco, plus a guppy. Filter is a Fluval C4 HOB with the stock foam/poly pad and Matrix replacing the carbon. Substrate is pool sand, with Flourish root tabs. I also dose 2ml of Excel daily and Flourish weekly.
Plants are growing very well with a pair of 8" clamp lights on the Versa-Top running 9W 6500K bulbs. I did have a bout of staghorn/hair algae recently, but that was before Excel. Otherwise, algae is nil.
With that said, this brown stuff drives me NUTS! It makes the water murky and settles down on all the leaves. I can remove it by installing a little Aqueon 10 HOB filter with just a 100 micron poly pad. For some reason the Fluval stock pad just isn't fine enough to filter it out. 24 hours with the added filter and the water is crystal clear and no more brown dust.
I have noticed that over time, as my plants have established themselves, the level of brown "goo" that builds up in the filter is much less (and the brown dust is much more). My theory is the bacteria aren't getting enough to eat and wash out of the filter as they "starve" because the plants are consuming much of the food before the bacteria can get to it.
I have another 20G tank with an Aqueon 20 HOB filter, stock cartridges, that is partially planted with about the same bio-load and light level. I did have a green water problem that UV took care of, but never have any issues with the brown dust and the water is always crystal clear. Since the filtration performance is about equal, my only explanation the theory above.