I am really intrigued here!
How on earth do you make a jar of home-made whatever, especially jam, last long enough after opening to develop mould? No chance in my kitchen!
Mind you, at son's house a part jar (of my soor plum and rowan jilly) got overlooked and ws found some time later absolutely encrusted with mould (probably due to casualness with getting butter in the jar learned in student days). As the mould was so thick as to have reached the half inch long 'grey hair stage, a certain amount of amusement was gained by using a fork to apply a parting and various forms of 'hairstyle'. Alcohol may have been involved. And afterwards, as there was almost half a jar in there, a half inch of the mouldy stuff was scooped off and the rest (sans lid) given a quick zapping in the microwave and subsequently eaten with no apparent ill effects.
edit: but we are all trained mouldy jam-eating professionals so don't try this at home, kiddies.