It’s mold.
Use a weak solution of bleach & water.
You may not get all the black out, but you can control any new stuff.
Always deflate & remove the float bags after each use. If you leave it in the boat, it restricts air circulation and allows the mold to take over.
Please, Jerry, it is not practical to
remove and dry float bags after each use. Yeah, they may get some black stuff on them, but actually they dry out before the next week’s use.
I had four Voyageur ultralite bags in a decked c-1 through 20 years of regular use, and I had absolutely no problems with leakage, smell, or anything else. I have had ultralite Voyageur bags in a tandem WW open boat for a dozen years. They are dirty in places, and I have quick-patched a few tiny leaks due to the bags flopping when on the car during fast driving. But the bags are still quite usable. Their dirty appearance probably helps keep the boat from being stolen.
Having to deflate and dry out float bags after every trip is just unnecessary. If you want to do it, please do not pretend that there is any compelling reason for doing so.
Just don’t get
any greasy black stuff on the grey thing.