I'm pretty sure this is Diadema Genius. I'm less sure about setosum but it does match what I'm seeing on the internet. At the time, I thought these were feeding on the coral but from what I've read, they feed on algae.

Camera:	Pentax Optio WPi
Date:	Tuesday, December 4th, 2007 at 9:13 PM
Exposure:	1/400 sec at f3.3
ISO:	80 asa
Lens:	Built In Zoom Lens at 6.3mm (equivalent field of view of a 38mm lens on a 35mm camera)
Photographer:	John Harvey
Scientific Name:	species related to Diadema setosum (Setosum Spined Urchin)
Tag:	Vietnam, underwater, sea urchin
Larger Image:	2000 x 1500
Raw Image:	2816 x 2112