This is all I could find (it was on a tractor forum)
I've heard the moly can build up and reduce the clearances in a rolling element bearing to a negative number. When that happens, the bearing self destructs. It doesn't happen every time but it does happen and when it does, it's the last thing anybody ever suspects for the cause of the failure.
Then I found this to counter the first statement
Virtually everyone of those references are from old technology from Industrial grade Moly at 7 microns. The largest percentage of lubricants fortified with Moly in todays industry uses the 3 micron Moly. Schaeffer's grease uses a Technical grade Moly at .7 microns. At that grade, the protective layer that is built is a micro-fraction of a human hair. It is not mechanically possible for this Moly to throw tolerances off in these bearings or result in corrosion. The "disulfide" aspect of Moly prevents this. Sulfur is an anti-oxident. The corrosion you refer to occurred from raw molybdenum in low quality products trying to compete by offering low cost "Moly" greases.