Use the hose to spray and rinse away as much moss, mold and debris as possible. Using the spray nozzle attachment on the hose may be a little more advantageous. The easiest way to loosen this is to use a pressure washer.
Use a soft to medium bristle brush to help loosen up any remaining debris if a power washer was not used.
Pour 1 cup of vinegar into a spray bottle and saturate the rocks thoroughly. If you need more, use only as much as needed. Too much vinegar can be harmful to plant life by lowering the pH balance in the soil. While vinegar is great for killing weeds, you may not want to use this near plants you don't intend to kill. Some plants such as rhododendrons, hydrangeas, azaleas and gardenias love the acidic soil. If possible, move the rocks to a better location where the vinegar won't disrupt the garden.
Allow the rocks to soak for about 15 minutes.
Rinse the rocks by spraying them with the hose.
Use the brush to remove any debris that might still be hanging on and spray again.