Whatever it is, its not being produced very rapidly so I'm not too concerned about it. The tank doesn't have much of an odor (besides the usual). Still, the gas production is pretty slow...so...who knows?
This part of the tank is pretty cruddy--the glosso is growing there (so I can't vacuum) and crud carried by my little stream tends to settle there. BTW, is mulm usually white? Whenever I stir up the substrate, white crud flies all over the place.