I found the same in my Deuce when it was only two years old. I noticed the older sealer was grey, I think the newer are black. I think they all do it eventually. I was able to look inside the Deuce tank and see that it all originated around the welds for the gas gauge wiring tube and the fuel bung and crossover tube. Cleaning welds is very difficult especially in a confined area.
When I removed the fuel petcock on the carb. Deuce, the attached fuel screen was covered in the grey matter above the reserve level which is expected. The filter screen is doing its job but needs to be reomved and cleaned often once you see the liner particles.
The injected models have a much more elaborate filter setup, but also cannot be ignored. It would be much easier to fix crap in a carburetor than clogged FI pumps, regulators and injectors.
This is all my opinion BTW.