Is it normal for some saltwater fish to hide between the rocks?

Yes. Many species of saltwater fish, including wrasses, blennies, and gobies, are known for their habit of hiding between rocks and crevices in the reef. In fact, some of these fish are even called "rock dwellers" because of their preference for this type of habitat. There are several reasons why saltwater fish might hide between the rocks. One reason is that they can find food and shelter in these areas. Rocks provide a variety of hiding places for fish to avoid predators, and they also attract small invertebrates that fish can eat. Another reason is that rocks can help to regulate the water temperature in the reef. This can be important for fish that are sensitive to changes in water temperature.