Some tend to think foxing is mold. They may not be the same, but are caused, and treated the same. Stored properly, it wont get worse.
Edit: This reminds me of a mold or "plume" question you would see about cigars. "Plume" was made-up-voodoo-urban-legend-tribal-"knowledge." It was mold, all of it. The oils in cigars didn't crystalize over time to form...its mold. I believed in plume from 1991 to about 2012.

Foxing has a designation and is real. I would just treat it the same.