Metal grease is used in the shaft seal section, but air mixed with Nisseki Mitsubishi Turbine Oil 32 (turbine oil type 1 ISO VG32) comes in contact with it. Will this cause any problems such as alteration?

There is absolutely no problem with mixing metal grease and other oils and greases.
For bearings that use metal grease, I also apply Bell Hammer to the base bearing races, balls, needles, etc., and then cover them with metal grease.