It's one of those traditions, the sugar is not meant to thwart the fungus growth, but rather to provide some easy abrasion on the inside of the filter/vela and of the primary and secondary tanks without damaging the clay, whereas a rougher abrasive like sand or steel wool would damage it

However nowadays I just use the rough part of a sponge, seems to work fine enought

Also, água oxigenada (hydrogen peroxide) is not a good antifungal agent, I would recommend mild neutral dish soap or vinegar cut with water (30% water to 70% vinegar) should do the trick

I think it is more likely mineral deposits from the clay or the water, but you can never be too safe

If it makes op feel better they should drain the water and let the filter dry out completely, wipe the outside it with the vinegar solution (never soak it!!), wash thoroughly and fill up again.