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Abstract Coastal darkening is emerging as a global threat to fringing reefs. While some reef‐building corals exhibit resistance to low‐light environments, the mechanisms behind this resistance, particularly the role of coral hosts, remain inadequately understood. Here, we investigated variations in underwater photosynthetically active radiation (PAR) and employed the Bayesian stable isotope mixing model (MixSIAR) to estimate the contributions of autotrophic (i.e., dissolved inorganic matter, DIM) and heterotrophic sources (i.e., particulate organic matter, POM, and dissolved organic matter, DOM) to the nutrition of the reef coral Galaxea fascicularis on the Luhuitou turbid reef in the northern South China Sea. Our findings revealed that the heterotrophic contribution to coral nutrition increased to 58.5% with decreasing PAR and that the heterotrophic contribution was significantly negatively correlated with δ13C difference between host and symbiont (δ13Ch–s). Moreover, we observed significant seasonal variations in the respective contributions of POM and DOM to coral nutrition, linked to the sources of these nutrients, demonstrating that G. fascicularis can selectively ingest POM and DOM based on their bioavailability to enhance its heterotrophic contribution. This heterotrophic plasticity improved the low‐light resistance of G. fascicularis and contributed to its prominence within coral communities. However, with a low‐light threshold of approximately 3.73% of the surface PAR for G. fascicularis, our results underscore the need for effective strategies to mitigate low‐light conditions on nearshore turbid reefs. In summary, our study highlights the critical role of heterotrophic plasticity in coral responses to natural low‐light environments, suggesting that some reef‐building corals with such plasticity could become dominant or resilient species in the context of coastal darkening.