Your best chance of getting a diagnosis is by asking your doctor to have this “grape” examined in a microbiology lab. The description suggests that it might be either mucus that has had a long time to compact and/or a fungus ball. It is possible to get serious fungal infections of the nose and sinuses. You need to see an ENT doctor. This specialist will examine your nose and throat. Other testing that may be done is to culture a specimen from your nose and a CT scan of your sinuses.
Whatever the cause, it should not be ignored or attributed to infection that will go away spontaneously.