Yes, you can substitute mozzarella for Halloumi. Both kinds of cheese have a similar taste and texture, so they will work well together in most recipes.  Mozzarella is a bit milder than Halloumi, so you may want to add more of it to your dish if you are looking for a more intense flavor.  Halloumi is also a bit saltier than mozzarella, so you may want to adjust the amount of salt you use in your recipe accordingly. Overall, either cheese will work well in most recipes that call for the other.