Corrosion *is* oxidation. And after some googling, I'm rather surprised to find that it is still legal to use cadmium in electronic products, as long as it is below 0.01% or 100ppm by weight. Cadmium is certainly *not* harmless.
On reflection, it probably is - the fact remains the discolouration (which is common) does not seem to affect performance, and does not warrant drastic removal action. Simply inserting and removing a plug should be sufficient.
I believe, like the other ports on the Pi, it is just steel.