it is not mold; it is plasticizers coming out of the plastic. I remove it with various means: alcohol, polishing with a very fine polish, Goo Gone™, naptha, and such.
The white gunky substances seen on the plastic keys of old, unmaintained typewriters generally isn't mold, but plasticizers coming out of the plastic. These can usually be cleaned off using simple household cleaning products or if necessary heavier cleaners (Goo Gone, alcohol, naptha, etc. - test these on hidden parts first to ensure they don't react with or destroy the plastic or remove the paint of the key letters) followed up by light waxes or polishes and buffing.