The answer to 'why is my paver mud' is that paver mud, also known as paver sand, can become saturated and turn into a muddy, slushy mess for a few common reasons. This can happen if the paver base wasn't properly compacted, if there's poor drainage in the area, or if heavy rain or irrigation has oversaturated the sand.
Paver mud can make walking on your pavers slippery and uneven, and it may even cause your pavers to shift or sink over time. To fix paver mud, you'll need to address the underlying cause - either improve drainage, re-compact the base, or replace the saturated paver sand. Depending on the extent of the issue, this may be a DIY project or require calling in a professional landscaper.