If you are trying to encourage an all moss lawn area then the best way to get rid of the grass clumps would be to dig them out. Moss prefers shade, moisture and acid soil and there are hundreds of different types of moss that prefer different growing mediums. Some like to grow on rocks, some mosses prefer moist ground and still some other mosses like to grow on the branches of living trees and shrubs. Using an herbicide to kill any grasses could harm the moss you want to propagate. Adding lots of water holding compost to your soil will help it to retain moisture and create a better moss making environment. Keep growing, Marianne Binetti