I to see a lot of forclosed homes with mold and it is common due to the houses being cold, dark, and damp. Almost 100% of the time if there is mold on the wall surfaces there is a great deal of moisture behind the wall. All the materials should be removed such as wall covering, "fur" strips or studs, carpet and sometimes cielings.Any materials left behind should be treated if possible. Leaving any material behind that is not treated is not recommended because of the mold that is not visible. Also keep in mind that removing material does NOT fix the PROBLEM, Moisture is intruding and that should be addressed, such as low grade, gutters, bad or broken drain tile, broken water pipes, bad shingles and etc. Some "contractors" are able to remediate but, a knowledable remediation company will know that you can't stop and the surface or even just in the area or room it was found. You have to inspect and treat the entire home in most cases.
I have inspected and repaired many homes with mold and can tell you first hand that the mold does not stop on the surface..EVER. To the Realtors showing and listing these properties remember this, if there is visible mold do not turn on the HVAC untill it has been cleaned and removed as it can and will spread through the entire house causing serious health hazzards. Also Mold and Mildew are the same. If it smells or looks like "Mildew" it is Mold.