The use of residual biomass from industrial seaweed processing in aquafeeds may represent a potential route for the reuse of residual biomass within a circular-economy framework. This proof-of-concept trial evaluated the effects of dietary supplementation with wet solid residue from the seaweed Kappaphycus alvarezii in Mugil liza over 80 days under a mixed-feeding system. The treatments comprised nominal wet-residue levels corresponding to 0, 6, 12, 18, 24 and 30% of the reference daily feed intake, equivalent to 0, 0.7, 1.5, 2.2, 2.9 and 3.5% of the dietary mixture on a dry-matter basis, respectively. No mortality occurred, indicating no detectable adverse effects under the tested conditions. Haematological data indicated a pattern consistent with hormesis, with the 6% and 12% nominal wet-residue levels associated with increased erythropoietic responses. Intestinal integrity and fillet nutritional quality were preserved; however, eviscerated yield showed numerical variation among treatments, with the lowest median value observed at 18%. The mixed-feeding strategy allowed a reduction in commercial feed input without compromising growth. Overall, the tested nominal wet-residue levels, corresponding to 0.7–3.5% of the dietary mixture on a dry-matter basis, were not associated with detectable impairment of survival, intestinal integrity or fillet quality. Future studies should employ randomized designs with replicated experimental units to further explore the potential of using K. alvarezii processing residue as a partial replacement for traditional feed components in M. liza production.