The understory is there to provide a function - it protects the sump from damage from speed bumps and road debris while also aiding aerodynamic airflow under the car. However, it is also known to trap moisture, which can cause subframe corrosion on these models. The advice from most Alfa Romeo specialists is to therefore remove the under tray, wax treat and rustproof the area around where it attaches to the car and then refit it, thus removing the rust issue but not exposing the car to unnecessary damage. If you can't do this yourself, it's no more than a couple of hours work for a garage so shouldn't be too expensive.