Do you mean apply a grass texture or add an actual grass deco(like bushes or shrubs)?
grass is static mesh. just make sure you take off it's collision
actually it is a static mesh. the deco layers can only use static meshes. the terrain just places them for you. and sense it's a part of a deco layer it can be turned off by the client using the setting "Foliage" it doesn't matter if the mesh is a rock or a human if it's used in the terrain deco layer it's Foliage to the game engine