Mold issue after tenant

[Mats Daniel Skrede]

Hello, 

We have some mold on the drywall after some tenants moved out. They told us that they had attempted to wash it away, but it came back. We got some mold inspectors to check on it and they told us that it is probably due to poor ventilation in the apartment and the mold appeared in this corner of the living room due to the isolation not being great, making it colder than the other areas of the apartment. They only did a visual inspection.

We have no experience with mold issues, so we are looking to see what you bright minds here think? Would we need to do a test inside the wall to look for moisture? 

And, would we need to replace the drywall, or should we be able to kill off the mold and be fine? We will of course fix the ventilation and/or get a dehumidifier.