I would not use acid on nylon. Strong acids literally dissolve nylon, and weak acids take a heavy toll. This is a specific weakness of nylon. In fact, it is one way you can identify nylon. Google it.Any chance that it is rust? On my slip there is a leak in the shore side water spigot and it drips rusty water on one of my dock lines. It is all rusty now. If it is rust, you need oxalic acid. Many products have it but Rit rust remover is probably the easiest to get your hands on.