Snorkeling north shore St. John and snapped these pics. Looks almost like a diamond blenny, but the head isn't yellow and this fish has another dark spot with neon blue center on rear dorsal fin, which makes me wonder if maybe it's a juvenile. So many fish have these spots when juveniles, then loose them later. The red/orange anal fin is curious also. Taken in about 2 feet of water over a Manatee seagrass bed with sandy, rubble strewn bottom.
Just a snorkeler,