Your Dual Function Metal Top is made from Cast Iron, so if it becomes damp or wet (and stays that way for a while) it will start to rust. To remove any rusting, remove it from the top of your EGG, coat it in oil, and physically heat it inside the EGG to about 200°c for approximately 20 minutes.
Alternatively, we would highly recommend upgrading your top to one of our new rEGGulators, which is better in almost every way! Find these over at the BGE shop.