The crab Trapezia bidentata is a conspicuous crustacean characterized as an obligate symbiont with branching corals, such as species of Pocillopora. These crustaceans have been identified as strict corallivores with a feeding dependency on the mucus excreted by adult colonies in the genus Pocillopora. However, like other crustaceans, individuals of T. bidentata have a nutritional plasticity that has not been described. We used an integrative approach, including traditional taxonomy and high-throughput sequencing of hypervariable region V9 of the 18S rRNA gene, to describe the mouthparts and the diet of individuals of Trapezia bidentata associated with adult colonies of Pocillopora verrucosa. The presence of setae on the ambulatory dactylus and maxillipeds, forming food brushes and combs, was evident. Sequencing and analyses of the intestinal content of the crustaceans found evidence of 18 phyla, mostly represented by Brachiopoda (28.1%), Arthropoda (31.7%), and Cnidaria (21.1%), but also small contributions (≤2%) of other animal groups such as Mollusca, Annelida, and Tunicata. In addition, a few traces (≤0.5%) of algae and fungi were identified. The feeding structures and intestinal content showed that individuals of T. bidentata are omnivorous and behave as suspension feeders and scrapers, obtaining nourishment from tissues transported in the water column and in the mucus and tissue of P. verrucosa. The results provide a clearer characterization of the role of T. bidentata, which is a crucial guard crustacean associated with P. verrucosa. We found both corallivorous and suspension-feeding habits, which demonstrates feeding plasticity, a positive characteristic for the survival of the species.