Free-living biofilms have been subject to considerable attention, and basic physical principles for them are generally accepted. Many host-biofilm systems, however, consist of heterogeneous mixtures of aggregates of microbes intermixed with host material and are much less studied. Here we analyze a key property, namely reactive depletion, in such systems and argue that two regimes are possible: (1) a homogenizable mixture of biofilm and host that in important ways acts effectively like a homogeneous macrobiofilm and (2) a distribution of separated microbiofilms within the host with independent local microenvironments.