Its call plastisizer migration.
Green gundge or green goo
*Green gunge (sometimes called green goo) is sometimes seen exuding from the ends of some older PVC insulated and sheathed cable. This is seen in some cables made in the 1960s and 1970s, but is not generally seen in modern PVC cables.
Its origin is the plasticiser used to provide flexibility in the PVC polymer compound. This is generally di-octyl phthalate, which over time or with excessive heat has reacted with the copper conductors to produce copper phthalate (hence the green colour) suspended in the liquid plasticiser. The material is of health concern, so should be handled with care - gloves should be used and waste disposed of properly.
Industry guidance is generally as follows, if green gunge is seen. Although there does not appear to be a problem with the electrical performance or safety of the cable itself, any exuded gunge should be removed as it can cause corrosion or affect the action of switches and terminations, potentially resulting in tracking / overheating. It can also cause cosmetic problems such as staining. The affected circuits should be rewired as soon as possible.
The original manufacturer of the cable should be contacted if there are any additional questions.*
Source - Dr Jeremy Hodge, BASEC