The answer to 'why is my stone walkway mud' is that a stone walkway can become muddy due to poor drainage, soil compaction, or erosion. Symptoms include standing water, soft or uneven surfaces, and tracked-in mud. Solutions may include improving drainage, adding gravel or sand, or resetting the stones.
If the problem is minor, you may be able to fix it yourself by adding more gravel or sand to the walkway. However, for more serious issues like poor soil conditions or structural problems, it's best to call a professional landscaper to assess the situation and provide a long-term solution.