Is there any kind of a film of this brownish red stuff? Can you collect it in a turkey baster? Is it almost black when out of the water? If the answers are yes, then it is likely to be cyno bacteria (often called cyno algae or just cyno) and you will need to get it under control quickly. I would recommend against stirring it into your sand if it is a dark, purplish red (tan/orangish red is likely just brown algae). Cyno in itself it is not dangereous but it will cover and smother everything in the tank and is a general mysery to eliminate naturally.
The natural treatment is to remove as much of it as you can and do a major water change. If you can remove some of the rock without a problem, remove it and rinse it well with fresh saltwater (you may want to use a soft brush as well) then put it back into the tank. Continue very frequent water changes along with more manual removal.
I have never had major success with the natural way. The chemical solution I have successfully used includes the initial cleanup and water change but also includes putting 1/2 the recommended dosage of ChemiClean (this product only!) on the back side (the side facing the waterflow) of a cascade filter filter pad or poly filter (if you are not using a cascade filter, pickup a cheap one that can conveniently fit on the side of the tank, the more flow the better). Putting the treatment on the water flow side keeps much of the chemical out of the tank but exposes the water to the antibacterial chemical. I have had to retreat two tanks when I used 1/3 the dosage but I have never lost critters or corals with this method and have been 100 percent successful (usually with one treatment).
I have had outbreaks of cyno during the first year in all but three of my tanks and the only recurring outbreak after this treatment was after an experiment with AZ9 to reduce nitrate. The resulting outbreak from THAT product required two treatments.