The answer to 'why is my stone walkway disease' is that your stone walkway may be suffering from a buildup of moss, mildew, or other organic growth. This can cause the stones to become slippery and discolored, detracting from the overall appearance of your landscape. Common causes include excessive moisture, poor drainage, and lack of proper maintenance.
To address this issue, you'll need to clean the walkway thoroughly and take steps to prevent future growth. This may involve pressure washing, applying a stone cleaner or sealant, and improving drainage in the area. In some cases, you may need to replace severely damaged stones. For more complex issues or if the problem persists, it's best to consult a professional landscaper who can assess the situation and provide the most effective solution.