Moss is a simple plant with no root system that grows in a dense mat when there is an abundance of moisture and very little light. When moss grows against boulders, it can be considered a very attractive plant for landscaping; however it can be undesirable when it grows against decorative stone or brick. Fortunately, you can easily kill moss with a solution made of water and sodium borate, more commonly known as borax.
Mix ¼ cup of borax with a gallon of water in a gallon bucket.
Dip a scrub brush into the solution.
Work the liquid borax solution into the moss with the scrub brush until the moss is saturated.
Repeat as needed until moss dies and turns brown.
Peel dead moss up from its bed and discard.