You'll have to get a certified mold re-mediation contractor out to look at this. It may be as simple as treating the specific area, or as complex as removing the drywall with a somewhat surgical HEPA filtered process that involve disposable spacesuits and respirators.
I experienced this process a few times and once a re-mediation contractor takes a look at the site, they'll be better able to assess.
Obviously, meticulous removal and subsequent drywall replacement and cleaning of the property can be very costly, likely in the many thousands of dollars. The real danger is to handle cleanup in such a way as to not spread mold spores throughout the property though ductwork, plumbing chases, etc. Mold spreads easily by spores and takes a fair bit of education and equipment to remove without spreading.
I attached a link to IDPH factsheet on mold removal.