The wear you see is not the tennifer coming off. Tennifer is a trade name for a nitriding process which uses cyanide for a medium at extreme temperatures to create a surface (and subsurface) hardening by bonding nitrogen with the steel at and near the surface. It also creates an excellent corrosion resistant property that will not be rubbed off, no matter how many holster presentations you perform. You would have to grind into the metal to get beneath the tennifer treated barrier of the steel.
The wear you see is the phosphate and or other surface coating Glock used (depending on when it was made) that mainly serves to make the slide and barrel black in appearance. When it wears off, it will not hurt the corrosion resistance or hardness of the tennifer underneath. You may wish to have it coated with another coat of whatever if the wear bothers you, but it is completely unnecessary in terms of concerns about corrosion resistance, unless you believe that tennifer is not good enough by itself (but it pretty much is). Personally, since you use it so much and it is not some trophy dispay piece (which no modern gun should be), I would leave it be and take pride in the wear or not care about it.