Hi, my cousin juste coughed up a big (2cmx.5cm) piece of greenish, light brown stuff. otherwise he has the symptoms of a common cold. should we worry?

Hard to say: But it's possible that your cousin just coughed up something that "went down the wrong pipe" from a bit ago. Otherwise this could be nasal or sinus secretions from the back of his throat or that traveled to the trachea (wind pipe). I would encourage him to keep an eye on things but to seek help if he has significant respiratory symptoms like coughing, shortness of breath or persistent fevers.