Depends, if it's a green base, it could just be from that. Does it have the manufacture date on the bottle - or how long did the other person have it. You could test it out on someone you know as a patch test and see what happens. But I would say it's just from where air has gotten onto the exposed spray and turned it green around the cap - as I said, if the tan solution is a green base then it'd turn greenish when in the air after a while.