[Lord of Hog]

Copper oxidises green just like iron oxidises red and aluminium oxidises white. The warm and sometimes damp environment under the bonnet is pretty much a guarantee that this will happen to at least some degree.

If you've got coolant in the V then have a look at that thermostat and associated pipework. Mop up all the damp stuff and then see if the dried up bits seem to lead anywhere. Remember that water runs in everything but straight lines!