I don't know how useful or accurate this is, hopefully someone will correct me if I have a bad method.  I have seen these odd spots on rubber and plastic parts in my machines.  If the rubber is otherwise ok, I have been taking an exacto knife and very carefully and gently scraping the spots off.  When scraping I am using a magnifying light from a craft store.  I am using silicone oil or silicone grease on rubber to hopefully preserve these parts a bit.  Silicone based oils are better for rubber and plastics from my reading.  I have only needed to replace one rubber sheathing so far, and used modern shrink tube for this.  It does not look completely period correct, but this particular wire was fortunately out of sight normally.  

I have seen video of typewriters with very badly aged rubber, I have been lucky with my devices that the rubber is generally in good shape.

Now, this is addressed to you and anyone else who may have information. Flipping open the lift panel, I saw a cable with white spots that I think I’ve read about before, as I’ve seen it on the cables in one of my electric typewriters. It’s related to moisture reacting with the rubber hosing, right? Or wrong, lol. How would one go about replacing the rubber? Is it very involved? Does it hurt to leave it (or them, if I see any other rubber with the same affliction) like that? I don’t plan on doing it quite yet. I think I’ll be taking this one small step at a time. Thank you for your time!

Fabiolah