Home inspectors should point out signs of excessive attic moisture and it is common for that moisture to cause mold in attics, especially in homes constructed from the late 1950s through the late 1970s. However, they are not required to specifically identify mold. See the Standards of Practice of the American Society of Home Inspectors for what is and isn't included in a home inspection. That being said, what the inspector saw may or may not be mold. It's more likely the early stages of roof rot, decay and delamination of the roof sheathing. If this is the case, the solution is twofold. First, deal with the cause, which is inadequate attic ventilation. To improve attic ventilation, make sure your home has continuous ridge and soffit ventilation. Many older homes simply don't have adequate ventilation and as such, moisture gets trapped in the attic where it condenses on roof sheathing and causes decay.
Secondly, you should ask your home inspector if the mold in the attic and roof sheathing decay is serious enough to warrant replacement of the roof sheathing, something that could easily double the cost of your next roof replacement. If so, you may want to renegotiate the purchase price accordingly.