Not an expert on this but most molds can produce spores to spread themselves. So I would keep the box closed to prevent contamination of other paperwork.
Molds need oxygen while slowly digesting their food: your vintage paperwork). Putting your box in a jar under a pure nitrogen atmosphere would do the trick to prevent further growth but is not very practical, I admit.
Also try to lower humidity to slow down further growth.
Hope this helps.
Marco