Ouch, ya this is verdigris, typically consisting of copper carbonate but can also contain copper chlorides (by the ocean), sulfides and sulfates (pollution). Copper oxidation will start off as turning darker shades of brown, eventually going green. What's strange is in cables the whole oxidation phase - from brown to green - tends to happen exceedingly fast, seemingly skipping the browning all together as verdigris from copper oxidation typically takes years to form under average air conditions. Copper carbonate is not electrically conductive - unless melted - and this is compounded by the skin effect of cables. How audible is copper oxidation leading to copper carbonate deposits? Not sure, as I've never A/B'd nor have I knowingly used copper cables containing copper carbonate....although I can guarantee I unknowingly have. But if you can't hear it, clearly that's all that matters.
What's really strange here is that Lionheart is supposed to contain gold-plated copper. Clearly not 24K pure gold but even at, say 10K, your cable shouldn't have oxidized this quickly. It's not like wherever you were vacationing folks can't own less than 24K gold or else it tarnishes within days, so this is very peculiar.
If not going all the way to iem connectors, nor to Y-split, and considering it's on both channels (nicked both) air leakage is very suspect. With all this added up, skin contact makes the most sense. Acetic acid/salt from your skin will oxidize copper faster than wet and salty air, and this happened very fast indeed. But the insulation still needs to be broken down by something to allow seepage. Insect repellent, sunscreen, lotions, but especially LUBES could possibly break down PE over prolonged exposure, but this is a lot of guessing...and a little joking.
Unfortunately, I don't think you/they'll ever know exactly why this happened - depends if they want to CSI this or not - so I think the bigger question here is why gold-plated copper oxidized exactly like copper would? How much copper was in that gold-plating? Or....I don't even want to say it.