You most likely have some sort of water leak or high humidity levels in your home. What ever you can see is 10 times that behind the wall.
Get a fee estimate and pro opinion before you decide what is best for you and your family.
For more information:
Read "Should I be concerned about mold in my 87 year old property that has plaster and lathe walls? There are a few areas of water damage." a Q&A answered by Harold Remlinger.