Laptop Charger - Play-Doh like consistency

I've had my mac book air and my the corresponding charger that came with it since 2015 and I keep care of everything I have considering nothing has broken, until recently the cord near the end of the charger has turned into a sticky sort of substance and even started to peel off, as I peel off to properly rap I feel the plastic cord that has just come off and there appears to be a green like substance I presume is the glue, however, the plastic cord feels like children play-doh I am even able to mold the cord into a ball like gum as if the glue or whatever substance is somehow in there is eating away at the cord. Pictures are below of what I am referring too. Just looking for someone to help explain why this would even happen in the first place.