It’s not unusual to see this kind of gang activity while snorkeling, and what they’re doing is hunting. Their prey is small fish that take sanctuary in coral heads and among the rocks.
This bunch of hunters is dominated by Blue Goatfishes, easily identified by their blue bodies and yellow saddle at the base of the tail. There’s also a Bluefin Trevally and Pacific Trumpetfish toward the bottom of the photo and, near the top of the photo, a Peacock Grouper with a Whitemouth Moray Eel curling below it.
Eels are popular members of these hunting parties because they can wriggle into the smallest spaces, flushing out prey. The goatfishes perform similar work using long, white barbels below the chin to probe small spaces in the hopes of disturbing a meal. Other fish tag along hoping to be beneficiaries of this work by being the first to snag any victims that get flushed out.