If they're acrylic canvas
(like Sunbrella), I wash mine in the washing
machine at the marina. I have not had that particular problem, though, and you might be able to hose it off. If the fabric
is chafing, that is a different problem, and sewing some chafe patches on the inside may help. [My experience here is that although you can glue patches on, if you want them to stay on, they need to be stitched in place.]
If the fabric
is polyester, I would machine wash that as well.
However, if it is pvc, I would hand scrub it, even try scouring powder. If it is pvc, the actual problem may be different, though, the heat from the metal where it rubs may liberate the plasticizers more rapidly than the rest of the fabric. If they don't clean up that way, you can try a light rub with acetone, but basically, the stuff's on its way out.
I'm not sure how yours remove, perhaps have to slide it off the lifeline. Mine are all tied on. Yours might be velcroed, zipped, or simply slid on.
Good luck with it.