A mall across the street pretty much universally increases the value of existing retail in the area due to increased car traffic. But the mall across the street shutting down or converting to non-retail use would fit.
Not universally true. Depending on points such as competing stores in the mall and the parking situation, number of customers can decrease for the store outside the mall. For example, you are a cellphone store with no parking and the mall will have 3 cellphone stores and a giant parking structure, the mall will hurt your business.