Aquarium magnets can become brittle over time and crack. This will expose the metal to the saltwater, which will inevitably cause it to rust. The metal will leach back into your aquarium and potentially cause many unwanted side effects. As a temporary fix, you could cover the cracks in the magnet with silicone or epoxy, but I highly suggest you start looking for a replacement immediately. Fortunately, most magnets aren’t very expensive and the hobby is full of them.