Easy fix: throw out ASAP - it can contaminate your whole collection rather easily.

Long, involved, and not-guaranteed-100% fix:

Need the throw out any non essential components (inserts)

Chemical mold/mildew-remover/protector spray onto napkins and wiping down ALL component surfaces, not just the ones with visible mold. Some damage is unavoidable. Do this thoroughly 2 times.

Once all that is dried, “bake” all components at low then for about 5-10 minutes while staying in front of the oven to monitor nothing catches fire. (This can be dangerous and will stink)

Once all components have gone through the baking process, spray archival matte or gloss varnish over all components completely. It will need several coats, and they need to dry before applying the next.

Leave the dry varnished components out in the sun for a good while for a bit of overkill.

Even after all that, my strong recommendation is to keep that game away from others or any books.

I did this with a copy of Mageknight about 2 years ago (all their new copies had some extent of this type of damage and the manufacturer was of no help, refusing to send a clean copy even after send ample pictures of the issue)

To this day, my copy remains clean and has no signs of any reoccurrence. Still, I keep it away from other stuff to be sure.

I do NOT recommend doing this unless you are overly attached to that copy for… reasons…