I have quite a few old slides which show the same pattern. I've always assumed the marks are fungal mycelia, which appear to be embedded in the emulsion. So far, I have not found anything that can remove it. Including alcohol, as recommended in the link, above. Notice those instructions say:
"There is no satisfactory method for complete restoration once fungus growth has etched or distorted film emulsion."
I start shooting in 1970, so some of my oldest Kodachromes may have been coated with lacquer. I did not try to determine if my slides were actually lacquered, or not, nor have I tried removing any lacquer that might be present.