What happened here?


This is a Rhodium connector. Pulled the plug out to rewire and this looks like brown dust in one side of the plug. Rust?
Its not very well made if this happens.
rust on a rhodium plug is a bad thing. Surely it didn't get wet!
It looks to me to be copper.

Yup, the copper underplate. 
It looks like the copper is coming off the parent metal on the left and the rhodium clad is holding up on the SAME parent metal on the right. Brass, iron, nickel they electro plate it with what ever YOU’LL buy.. Just a cheap knock off, with a bad plating job.

NO they both didn’t start out rhodium plated, just the one on the right..

The Viborg connectors are Rhodium over copper. (Cerious Matrix). It probably is the copper, strange how it was able to pulverize like that from underneath the Rhodium coating. 
Sorry OP that's NOT what the picture shows. There is an alloy under the copper splattered on the clear insulation on the left, it sure doesn't look like copper. It looks like a knock off like I said. Rhodium will stick to plastic. Copper will stick but you have to prepare the surface.  Just because it what suppose to be made of copper doesn't mean it IS. I mean it LOOKS like iron or nickel or something else other than copper like on the clear insulation.. I see no rhodium,,

You ask I answered.. From what I can see. I have no idea WHY you would use rhodium. on a hot (at all) and surely not on the common or ground EVER. Its is for corrosion ONLY. Not a great conductor over copper. If it were submersed in sea water.. That would be a place to use rhodium.. WAY to hard.. It's used to help copper hold a spring BACH element too..
Kind of looks like maybe some type of contact treatment/enhancer was sprayed into the contact body. Maybe the stuff has gone bad, deteriorated . What piece of equipment was the IEC connector used on? Maybe a power amp? Poor contact pressure will cause heat and maybe heat caused the  contact treatment/enhancer to breakdown causing the discoloration inside the connector body.
It was connected to the amp.
jea48 - Plausible. Looks like an inferior connector, maybe defective from the start. My suggestion, don't use Rhodium ever for hi-fi. Silver all the way is so much better.