Hmm....that's weird since its stainless and aluminum I believe. Granted it's called "stain-less" but generally does not oxidize but it can happen.

Do play by the sea, or possible leave the headcover off during play or maybe put the putter head wet into the cover at the end of a the round?

No, I live in the Midwest.

And don’t necessarily play in a lot of rain.

Following up with pictures...