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If I am not mistaken, that is A S&W Scandium alloy frame. The metal is mostly aluminum with Scandium added to strengthen it.
Water will not bother it. The anodizing is just for looks and, since this part is covered by the grips, does not affect the gun in any way.
It would not bother me. For all I know, the fixture that holds it during anodization may cover the non-colored area.
Enjoy shooting your gun.