When Mello-Roos districts are formed, a Special Tax Lien is recorded against each property in the district. This allows the Mello-Roos district to use the value of the property as security for bonds that will be issued to finance the improvements and services.

As a property-tax related lien, it has priority over all other liens on your property (such as your mortgage). So, if property taxes are not paid, and the house is foreclosed on, the first parties to get paid would be the Mello-Roos district and the County. Your mortgage holder would collect money next to pay off the balance due on the mortgage.