As you can also read about in the Schooling species there are different types of fish aggregations. – the fish that hang out together more loosely – and the fish that fish that form tight groups and move together almost as one large organism.
The fusilier and the sergeants in the book, are most often in the middle or top of the water column 5-10 meters outside the reef. Thus they have no place to hide in case of predators approaching. The group is the safe place, and that is why they ensure to stay close together as soon as they feel a threat nearby. But unless they are on the move from one place to the other, they tend to stay close but unorganized in their swimming.
They seek shelter in being many. And they have something to fear. There are several predators that would not mind taking a bite (see some of the Silvery fish in the blue in the book or here).
On the inside of the reef you may find a pale variant of the sergeant that seems to be lost from the group and lonely. That is not the case. It is just a Blackspot sergeant (see the black spot on top of the caudal peduncle – the base of the tail – in one of the images above). And they live solitary.
Though placed on the next page in the book – the Striped Mackerel is also forming schools of silver – and they are often forming the most beautiful shapes as they move as one massive body through the water, all with the mouth wide open filtering zooplankton from the water through their gills. – stay still and they may come close.