Damp air, outside garage probably.
I would say its only gone down from the ends a little way, and would cut the plastic back to see how far.
But it could be a good excuse to get some new, but looking at your system, 135, 82, sbl’s, then i would say be a bit careful with that, as you dont want a bright cable, as it might upset your system, as i found with sbl’s, they prefer a more heavy sounding cable, to help the bass out, rather than the treble
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I know that my garage is very damp inside at times - but it has no seal around the up and over doors with quite a gap under them, and one door at least is opened creating a large air exchange area with the outside on average twice a day.
The other very significant question of course is whether a surface layer of verdigris on the copper conductors has any effect on the sound - and if it does, whether for better or worse? I doubt it would have any significant effect on the overall electrical characteristics. You would have to compare to know if there is a difference in the sound. If considering replacing I would suggest seeing if you can have on a trial basis.
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Good point about whether it makes an effect, it still sounds good, but without anything to compare to…
Oh come on folks it’s trashed. Just buy some new speaker cable. This isn’t going to turn into one of these crazy long threads is it? Copper oxidizes it it looses some of its conductivity. Simple fix, buy new wire.
Oxidation is only on the very surface. I doubt there is any measurable change to resistance, inductance or capacitance of the cable. Are you so sure it has a negative effect, or any audible effect at all? And if it does have an effect, how do you know it doesn´t improve the sound?
I guess I’m just wrong. Probably best to stick with your old oxidized copper.
Oh and @Innocent_Bystander might want to shore up those garage doors, mice are always looking for a nice home and love eating insulation. 
The point I was making was not that the cable is perfect as it is, but simply I’m not sure if anyone knows for certain what, if any, effect a surface coating of verdigris has on the sound…
Yes, I’ve been meaning to, but low on priority list. Mice are a problem where I live - but they don’t need a half inch gap to get through!
It can cause pitting on the surface of pipe, so i guess the wire will also get pitted.
But like i said i would just strip it back and re make the ends, or just buy new
Indeed it can. I have an ancient CB HiCap here that had nothing obvious wrong with it but didn’t sound great compared to an Olive one. The problem turned out to be corroded wiring at two crimped terminal lugs! This is multi-strand tinned copper wiring so it was dull grey rather than green oxidation.
As for the OPs cable; it could be stripped back, cleaned up and shrink wrapped, rather than binned. Shrink wrapping the ends is good practice anyway and would help prevent oxidation.
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Corrosion or more usually oxidation really only requires 3 things. Take one away and it tends not to happen. Think of it like the " fire triangle "
Something to corrode, iron, copper etc.
Oxygen. Even better if the oxygen likes to combine with the thing that corrodes.
An electrolyte, usually moisture. I.e. damp.
Iron in an oxygen rich, but dry environment corrodes/ reacts very slowly. Add an electrolyte and things take off. Make that electrolyte salty and even better.
You get the idea.
Yes there are other corrosion mechanisms that don’t involve oxygen ( iron, at elevated temperatures in a pure co2 environment for example,) but as 21% of our breathing environment in oxygen, this is the one we are most familiar with!
Give it a go with yours IB and let us know your findings!? 
I always use a flux cleaner and then heat shrink over a soldered joint at work. It also adds a bit of strength and support as the test department can be a bit heavy handed.
I go along with shrink wrapping, how straight forward is it with Naim plugs?
The cable I referred to wasn’t a speaker cable, and it is long since gone!
Of course, where verdigris would likely be a problem is if it was on the contact surfaces of the copper if clamped in the plug as opposed to soldered.
You might be surprised how many mission critical connections are crimped vs soldered. A perfect crimp basically turns conductor and connector into one piece of metal where vibration and cold solder joint don’t happen. In audio crimped connections also sound better because of the lack of solder. Of course not everything can be hand crimped
https://technology.nasa.gov/patent/LAR-TOPS-52
Indeed - but imperfect crimping is quite another matter…
Thought I would give an update since my original post. I have borrowed some WH N2 cable, not NAC A5 that I posted about but very similar by all accounts. The sound from the N2 either tells me it is a far better cable or the corrosion on the A5 is having a pretty bad impact on sound quality. Not a scientific test, but enough for me to say if corrosion is present in the way my cables have it is sadly time to replace them.
Thanks for your thoughts everyone.
Just as bad maybe worse is imperfect soldering
Could even be both!
Or something in solder / flux? Could be possible