Mold in wall

Hey guys, we just found that one of our waste pipes in a stairwell wall had a four foot long crack running down it. The wall was opened up to get to the pipe on the stairwell side and the problem section of pipe has been replaced. But there is black mold running up and down the area around the pipe from the time it was leaking (the floor above wasn’t used much until recently when we noticed the problem). The wall in question is between the stairwell and one of the floors, plaster on one side drywall on the other. Has anyone been through this and have advice how to proceed? Should be looking into licensed mold remediators? Suggestions are welcome.