If they do not come of with a quick rub with metal polish, they are most like corrosion occuring under clear anodizing. If that is the case, the only way to completely remove those greebly tendril looking stains would be to remove the anodizing through mechanical (buffer when and compounds) or chemical (lye based oven cleaner) means, then polishing the cranks with polishing compound. Just note that if you do so, you will have to maintain the cranks with periodical cleaning and polishing as the bare aluminum will corrode much easier that hwen it had the clear anodizing. It's not really too bad if you do not ride in very salty and wet conditions regularly.

Chombi