Most ocean life is opportunistic. They will eat just about everything. However, they prefer different types of foods. Tangs are omnivores, but prefer (and need) greens in their diet. Make certain to offer nori or spirulina during feedings and he won't pick off other life. But even though, predation can happen no matter what.
Just this week, I saw my pallet tang eating a mexican turbo snail. Actually swimming with it still in the shell across the tank while eating it (being chased by the Powder Blue who apparently wanted to see what he was munching on.) It just happens.