The answer to 'why is my retaining wall brown patches' is that the brown patches are likely caused by mold, mildew, or algae growth. This can happen when the retaining wall is not properly sealed or maintained, allowing moisture to accumulate. The brown patches may appear unsightly and can even lead to structural damage if left untreated.
To address this issue, you'll need to clean the retaining wall thoroughly using a solution of water and bleach or a specialized cleaning product. Be sure to wear protective gear like gloves and goggles during the cleaning process. If the problem persists or the wall is severely damaged, it's best to call a professional landscaper or masonry contractor to assess the situation and recommend the best course of action.