Cable plasticiser reacting with copper.

[Strima]

That's quite a bad case, I would be rewiring that lot.

 

[Marvo]

I agree with Strima. Usually I'd consider green goo just as an annoyance that wouldn't warrant rewiring if IR tests were up to scratch. In this case it's actually dripping and there's lots of foreign debris collecting which in itself is going to cause additional IR problems so I'd recommend a rewire.