The simple way would to be pour near-boiling water on them. Additionally, a 25% diluted common washing bleach will do the trick with the moss. The boiling water will kill the grass, but it will come back.
Finally, a pressure washer works well if you have the availability of one.
my hubby uses hypochlorite which he buys from farm supplies store farmers use it for cleaning milking parlours and it works great on our block paving. Its not expensive either which is good.