A little pressure wash might help.
I might be mistaken, but almost looks like this could be some extractive bleeding.
Tannin is a natural extractive of Cedar, Redwood and most other dark woods. It migrates to the surface of the wood when water is present and leaves an ink-like, coffee colored stain or overall darkness on the wood after the water evaporates. Tannin bleeding is unsightly, but not damaging to wood or any coating on the surface.
Tannin (or Extractive) Bleeding: Redwood, Cedar and Douglas Fir are examples of wood species that contain naturally occurring, water soluble chemicals that tend to migrate to the surface with exposure to extracting agents such as water. After rain or heavy dew, coffee or tea-colored stains may appear on the wood surface. If the wood is coated, discoloration of the coating may occur when tannin extractives are dissolved into the coating solution by water, reach the surface and remain as dark, coffee-brown stains after the water evaporates. This reddish-brown bleeding is unsightly, but not damaging to the coating.