The answer to 'why is my deck mud' is that a muddy or damp deck is often caused by poor drainage or excessive moisture buildup. This can happen due to issues like improper grading, clogged drainage, or high groundwater levels around the deck. Symptoms include standing water, soft or spongy boards, and discoloration or mold growth.
To fix a muddy deck, you'll need to improve drainage by regrading the soil, installing drain pipes or a French drain system, and ensuring the deck is properly sealed and waterproofed. This may require extensive work, so it's often best to call in a professional landscaper or deck contractor to assess the issue and provide the right solution.