Can Galvalume be near (around) Mortar?

Interesting. I am not sure where you saw that or if it's something I wrote or someone else wrote. However, my take on it is to try to not have them touching one another or, if they must, separate them with sealant. For example, we cut reglets into masonry walls and insert flashings. I would put some sealant into the reglet and then insert the flashing into the sealant bed. In any even, keeping the area of the joint free from a catalyst such as moisture is also a help. I would also in the example above bring the roofing underlayment up the masonry wall to create a separator between the metal and the masonry.