Lay something like a painter's drop cloth over the plants, so they won't get splashed with anything. Then use a detergent (like Dawn or another soap that will cut grease), and work on the stone with nylon scrubbers and brushes. Don't use anything with metal, because if the stone marker is old, it might destroy it. It will probably take more than one session to really get the stone clean. After you get the initial dirt/moss off, you can come back with another solution that has some bleach in it, and repeat the scrubbing. There is a man here in TX who does this for headstones from the 1800's, and he says it takes him 3-4 sessions of scrubbing to get one really clean.