Phestilla melanobrachia is a coral-eating nudibranch widely distributed in the tropical Pacific. Previous studies have revealed only two colour morphs of P. melanobrachia, an orange form preying on various orange Dendrophyllia and Tubastraea spp. and a dark green form preying on the black Tubastraea micrantha and Tubastraea diaphana. Both colour morphs live in relatively deep waters (15–25 m) of exposed shores. We conducted morphological and molecular analysis, which revealed a beige colour morph of P. melanobrachia that feeds on the hermatypic scleractinian coral Turbinaria peltata in shallow waters (<10 m). The three genera of corals all belong to the family Dendrophylliidae, but Dendrophyllia and Tubastraea are azooxanthellates whereas Turbinaria is zooxanthellate. Consistent with their diet, the beige colour morph has a high abundance of dinoflagellate cells in the ceratal tips, whereas the other two colour morphs contain only few or no dinoflagellate cells in their cerata. Our finding of this new colour morph extends the knowledge about the diet, ecological niche and potential ecological impact of this species of corallivorous nudibranch.