Coral reefs are full of life, but many species are not overly keen on being seen. If you’re armed with toxins or sharp teeth you can casually float around, or cruise the reef looking for prey. For other creatures there is too high a correlation between being spotted and being eaten. The result is that many spend their life in the shadows, having chosen to slip between the cracks.
For the photographer, the closer you get and the harder you look, the more complexity you will find. Almost every anemone, or coral growth has its own micro ecosystem living around it. You just have to find it.