Spray the rocks. Using a garden hose, spray the rocks to remove dirt and other debris. Adding the garden sprayer attachment can give you some added pressure. If possible, use a pressure washer to remove the moss, grime and dirt. Pressure washers can be purchased or rented from your local garden center or home improvement store.
Scrub with a brush. It may be necessary to use a scrub brush to loosen stuck-on dirt and debris. You can use a few drops of mild dish washing soap such as Dawn mixed with warm water. Dip the brush into the Dawn/water mixture and scrub the rocks.
Use vinegar. Fill a spray bottle with one cup of vinegar and saturate the rocks. Allow the vinegar to soak for about ten minutes and then rinse with a garden hose. Be careful when using vinegar around plants because it can lower the pH balance in the soil. If possible, move the rocks to a location away from plant life.