Never seen my emerald eat anything other then algae and coralline. He will pick at my zoas, but if I watch him closely, he's just picking between the polyps. If he does manage to grab a polyp(large and clumsy) by accident, he immediately spits it out. Pretty sure this is how the nuclear green palys made it to the other side of my tank.
I have never witnessed him picking on anything other then dead flesh on a coral or algae between the zoa mats. I will admit I only have 1 acan, but he hasn't even so much as looked at it, so far.