Detritus is a mixture of various things, from digested organics, precipitated salts, bacteria, zooplankton, and believe it or not, glass(from dead sponges and diatoms). This detritus needs to be removed, either via a protein skimmer, mechanical filtration, or ingestion by organisms. There are a couple different ways to acheive this.
First, make sure you're protein skimmer is functioning properly, preferably a wet skim as it removes larger particles better. Second, add a filter sock to either your sump or attach one to a pump in the tank. Thirdly, use a turkey baster or pump to blow the detritus off the rocks and into the water column to be eaten by corals, feather dusters, sponge, and other filter feeders or filtered out by the sock and skimmer. Try to do this on a regular basis. Your live rock will be much healthier this way. If you can, try to increase the flow or utilize the flow in the tank differently, inorder to keep this debris floating so it can be processed.
