A coral can resemble a branching plant or a textured rock. Its appearance can hide the small animals involved. Understanding the polyp gives the familiar reef scene a different starting point.
Begin with the living animal
NOAA explains that corals are animals. Many familiar corals consist of colonies of polyps; reef-building hard corals produce calcium carbonate skeletons. Many also have relationships with microscopic algae living in their tissues.
These details should not become a rule that every coral has the same form or lifestyle. Corals include different groups, and not every coral is a large colony.
Separate the layers in a description
When reading a reef photograph’s caption, look for whether it discusses living tissue, an individual colony or the larger reef structure. Those are related but different scales.
A distant image alone may not show enough detail to identify a species or establish the condition of the living tissue. Keep uncertain labels provisional.
Ask a better next question
Instead of asking only what shape a coral has, ask what evidence identifies it and what part of the organism is visible. Use a reliable local field guide for region-specific identification.
Compare this with seagrass, which really is a flowering plant. Similar underwater settings can contain living things with very different biological identities.