[Gaz64]

The rust on the pickup is the microscopic wear dust at the weight pivots. You can see it sprays out inside the bottom of the rotor.
There is a magnet as part of the magnetic pickup, hence the name. Guess where the dust ends up.
Clean it up as you feel, but LUBRICATE the weights with lithium, or a medium weight high melting point grease.
Your cap looks fine, clean it up with a wd40 soaked cloth. Sanding etc only creates larger air gaps.
MSD believes no lubrication other than their dry film is necessary. The rust does not occur if the weights are lubricated in the normal manner.
The same phenomenon occurs at a dry door hinge.