It'll leave the corals alone...but may munch on small fish and crustaceans that it can fit in it's mouth. All my sources points towards it being a reef safe fish.....just not reef fish safe.
Live Aquaria: "The Marine Betta is not commonly kept in a reef-style aquarium, but it is reported to be safe with corals and anemones, however, it will eat all small fish and shrimp"