Hi,

It looks like a thread algae to me.

Two questions:

1. Are you running 56w of 'Day' AND 56w of 'Natura' at the same time? I didn't quite get how much light you have...

If so, this is a lot of light for a 33 gal tank. 132w / 33 gal = 4 wpg which is a lot.

2. C02 by yeast does not always supply constant c02. This and the lighting may be contributing factors.

Hope this helps.