Ice222 did you get to the bottom of this one?
I've found a clump of brown candy floss growing in a part of my riccia which is right under the light. It can be twirled out on a stick etc.
Stuff I could do: stop the ferts (daily flourish comp), add the other bulb to make it 2 x 8 watt T5 14,000K colour, turn the lights off, stop feeding the fish for a while, plop it out and wash it in algae rid... the only other algae I've had in the tank has been a bit of green spot on the glass that appears occasionally and is currently on holiday. Never had diatoms or any other pains. My theory is that the 14000k light has little or no red colour that feeds algae and plants??
anyway - what of brown candy floss. I'd eat it but it doesn't look tasty. Do oto eat it?