Detritus and diatome algae. Totally normal to accumulate when the sand isn't moved much. Vac it off if it bothers you, but you don't have to at this level. More current or bottom dwellers that go through the substrate can also help.
Use pool sand in most of my tanks. Vacuum helps but eventually things just turn brownish. I tend to do cosmetic replacement of surface sand every 3 months and a more full replacement every 6-9 months. It's purely for aesthetic purposes but for some of us that matters and is OK. No sand will stay pure and light over the long term unless somebody knows a magic trick I don't.