Ignore the mold and cover it up with insulation? 

This solution is often adopted by builders who assume the wood will eventually dry thereby killing the mold.  To some extent they are right but time will tell.  The wood substrates does eventually dry arresting further mold spore colonization.  The problem is that even dormant mold spores can cause allergic reactions in susceptible individuals and health issues.   Any breach in the HVAC, plumbing, humidity or wire hole penetration into the building envelope can cause an aerosolize of these dormant mold spores.  This can create a potentially hazardous indoor environment leading to costly litigation not to mention health issues.