Both, in a way

Is Coral a Rock or an Animal?

Coral is millions of tiny animals living in a stone skeleton they built themselves.

The rock is the house, not the animal.

Step 1 of 5

Each polyp is a tiny animal.

The reef is stone. The thing that built it is a jellyfish that stayed put.

A coral polyp is a small sac with a mouth ringed by tentacles — a relative of jellyfish and anemones. Most are a few millimetres across.
Step 2 of 5

They build stone houses around themselves.

Each polyp pulls calcium and carbonate from seawater and builds a hard cup. The reef is those skeletons, accumulated over centuries.
Step 3 of 5

Algae live inside them and do the cooking.

Neither would manage alone in clear, food-poor water

Tiny algae live in the polyp's tissue, photosynthesise, and hand over most of the sugar. In return they get shelter and raw materials.
Step 4 of 5

The algae are the colour.

Coral's brilliant colours come mostly from those algae. Stressed by heat, the polyp expels them and turns white — that is bleaching.
Step 5 of 5

Reefs support a huge share of ocean life.

Coral covers well under one percent of the seafloor but shelters around a quarter of all marine species — because it creates structure.

A reef the size of a house may have taken a thousand years to build, one millimetre at a time.

The short version

Coral is a colony of tiny animals living in stone cups they build, fed by algae inside their own tissue.

Look at a piece of coral skeleton closely. The tiny cups are the individual apartments each polyp lived in.