Moss is a garden design element that creates a lot of atmosphere on natural stone steps. Adding it or allowing it to grow garden has an instant aged and established look. The main problem with it is that it takes special conditions to grow and maintain garden moss continually. And even then, it may still not grow on its own. See how to grow garden moss for a recipe that might help you get a start of moss in your garden or landscape.
Another thing I want to point out in this picture is texture. While the moss and all the other plants are green, they vary in texture and shade of green. Among these rock steps, there's not one flower or plant colors other than green and this garden is still interesting and beautiful. Yes, green is a color too.