The brown specs on the sand, rock and glass are most likely diatoms.. The most simplest of algaes.... Typically starts in your tank within the first 48 hrs of being set up. They will go on for awhile until your tank is "mature", sometimes being obtrusive and overwhelming and eventually being a slight nuisence as you wipe down the viewable sides of glass. Detritus is simple organic matter that is more of a khaki or light brown color. It is made up of feces, left over broken down food and other various wastes. Essentially it is a nutrient pile and a NO3 and PO4 factory... Adding more sand is a tricky thing... Not only will it cloud up your tank something nasty and be a real mess you have the potential to smother the existing life in the sand bed. It is best to do small portions at a time, and work the sand into the sand bed, not just pour it on top!! Good luck and happy reefing!