Use a gauge to first make sure the rope is within spec, also check the sheave for wear if you are getting some slip. I use a straightedge placed over the ropes to make sure they are equally seated. In some cases what up you are seeing is the core fibres breaking down and migrating with the internal lubrication to the outside. Is it like sticky grease or dried and crumbly? A good stiff brush has done me well in the past.