We observed and documented a white-mat like disease on the warm temperate coral Porites heronensis since 2013. It is characterized by the rapid development of a white microbial mat resulting in the mortality of coral tissue underneath. The prevalence might associate with warm temperature, sedimentation, or a common macroalgae Gelidium elegans that overgrow corals. We thereby surveyed the bacterial community of the coral, sediment, and macroalgae for the potential pathogens.