Animals.
The reason many people think of corals as plants is that most corals don't move and many of them are branch-shaped. Most corals rely on symbiotic single-celled algae for photosynthesis. Their function is to provide energy, so they are very similar to plants. The scientific community also had a long dispute a hundred or two hundred years ago, but there is no doubt that these are animals.
Although corals are animals, they contain symbiotic algae in their bodies. Even the color of corals is mainly provided by symbiotic algae, and 90% of their food is also provided by symbiotic algae. So we can say in a very complicated way that corals are stones, coral polyps are animals, and symbiotic algae in coral polyps are plants. They ddepend mainly on symbiotic plants to provide them with nutrients, then the animals create corals, which constitute coral reefs.