Leave it alone. Get a couple brittle stars and drop them in to keep it clean-ish. I doubt it is detritus, but more likely silt. Rocks do degrade over time and you get dust/dirt in the tank. The reason you don't see it in your tank is probably flow. That's just my $.02. If you are not seeing any Nitrates in your tank then obviously there is no excessive detritus decomposing.