Fish waste. Considering your relatively small fish load, your fish must crap a lot
.... the really light coloured sand just shows it up easily. Darker sand hides more. You might be over feeding but it is hard to say that what you see as a lot may really just normal.
Here's a quick shot of my tank bottom and it has been almost a week without cleaning and my cleanings are not very thorough... 1 Betta, 12 cherry barbs, 9 emerald catfish and assorted snails. I couldn't tell if there was crap or not as that is the mottled colour of the play sand I used.