The best way to clean natural stone, be it granite, marble, sandstone, or slate, is water with some PH neutral soap, like the one used in
We can also do it with water
and a small amount of ammonia
with a small amount of bleach, but ammonia is better, because it is not an acidic product, but a base and does less damage to stone.
What we should not do, never, is to mix ammonia and bleach, this mixture generates vapours that can be mortal when inhaled.