Matte, it is self explaining.
Plants need micronutrients, specially in pots. So we have to fertilize AND give them the micronutrients they need, from time to time.
Chlorosis is the most common (lack of iron mostly).
Carbon Dioxide (pollution) can turn leaves whitish.
I don't know if your problem is simply the Elm being a variegated cultivar, or if it is really a deficiency problem.
Do what Geoff suggested, cut off the white leaves and wait to see if it happens again.
Seaweed extract with sequestered iron.will help, but I don't think that is the "problem" I think there is nothing wrong with the leaves, if it was mine I would not be concerned at all and would just cut them off and see if they return. The leaf is showing a lack of chlorophyll but iI don't see it as a problem.