Yes, it's oxidation. I don't know of any true metal (i.e., excluding metalloids*) that can have a valence less than 0, i.e., that can be reduced beyond its metallic state.
Several can, but under exceptional conditions (soaked in pressurized carbon monoxide or worse chemicals), or certain combinations you're never going to encounter (e.g. cesium auride).
Anyway, what's pictured looks mostly superficial, you'll have to inspect a bit to see if it's deeper, pitted, cracked or otherwise compromised. If the latter, replacement is necessary; on the upside, circular MIL connectors like that never went out of style, but, neither did their prices.
So yeah, wire brushing to remove crust, a lubricant helps (not WD40, it's not a lubricant!).
I don't think contact cleaner helps particularly on bulk metal parts? It might not hurt. Definitely for the contacts, if nothing else.
If the parts aren't plated they can perhaps be soaked in acid or base, or complexing agents, anything that dissolves the oxides -- but be careful, this can be quite aggressive, leaving to a rough (matte) finish, and further corrosion as the chemicals are almost impossible to remove from microscopic pores in the metal (let alone gaps between plastic parts, if you can't completely disassemble them). More of a last resort, or an option on bulk pieces that you can further treat the surface of (so, mostly fine for, like, car parts, weldments, etc.). And needless to say this likely ruins any plating: zinc dissolves readily in acid, nickel not so much but pitting will lead to undercutting and ideally the part would be stripped and replated, assuming it was worthwhile to do so in the first place.
Compromised integrity can happen to certain alloys; aluminum, bronze and steel are pretty good for the most part, corroding mostly from the surface, give or take pitting. Zinc alloys are the most common exception, as a minute lead impurity leads to eventual destruction (the material crumbles apart). This mostly affects very old (pre war?) potmetal parts, before they were aware of this catalytic role or something like that; later parts should be pure enough not to be affected, but if not (bad batch?), it's always a possibility.
Oh, beware of cadmium plating, quite toxic especially to kidneys. Usually apparent as an otherwise fairly shiny metal surface, somewhat darker than steel, with greenish to yellow dusty corrosion on it. Use water to wash/scrub it off, prevent it from emitting airborne dust.
Tim