Sounds like you can power wash it and blow the deck apart...or cange the top boards altogether.
That is decay starting in the wood, some species of mold that is dinning out on your organic material.
Bleach is what I would recommend, and lots of elbow grease.
Check your deck to see if it is structuraly sound also. Poke a screwdriver around some joists and check your ledger board and connectors for damage .
Check deck saftey