Seaweed–coral competition is increasingly important as reef communities degrade, with algal turfs being the most common competitor. However, experiments assessing the impacts and mechanisms involved in turf–coral competition under field conditions are rare. We evaluated turf–coral interactions and their impacts relative to those of macroalgae by placing corals (Acropora pulchra and Porites rus) in contact with turf communities from territories of two species of damselfishes, with two common macroalgae, and with inert algal mimics as physical controls. After 13 d, turfs reduced coral photosynthesis by 31–59%, while macroalgae and mimics had minimal effects. After 24 h of contact, chemicals from turf surfaces suppressed coral photosynthesis by 8–29%, affected A. pulchra more strongly than P. rus, and the significant allelopathic effects during summer were undetectable during winter. Effects of turfs on coral microbiomes were variable; contact increased microbiome richness in A. pulchra but had minimal impact on P. rus microbiomes despite photosynthesis in both corals being strongly suppressed by turfs. Thus, mechanisms driving the outcomes of turf–coral interactions include allelopathy and vary with both the coral-alga pairings involved and between seasons. If seasonal differences are due to increased temperatures, impacts of turfs on corals may strengthen as oceans warm.