I have a Copperband in a mixed reef with various species of clams and everything form stony corals to zoas and palys. Never touches any of them. He occasionally takes a poke at a snail, but it's more curiosity I think.
Keep them well fed and their fine. Course that's easier said than done - they are one of the most difficult fish to get eating in captivity. Mine gets live blackworms , his favorite, and also enjoys frozen spirulina brine along with the frozen mixed food like Marine Cuisine, Emerald Entree etc. He'll pick at mysis too.
Beautiful fish, one of my favorite. Very active, always hunting for little critters to eat off the rocks. Love his long pointy nose.
That all being said...
That Purple Tang will very likely murder-death-kill a Copperband if you put one in now, especially in a tank that small. The tang is established already and likely rules the tank, as Purples do. The similar body shape rule comes into play here and the tank size doesn't help. Speaking of, I'm very much NOT a member of the tang police (currently trying to find a healthy blonde Naso for my 5 foot 120g), but a Purple Tang in a 46 gallon is a recipe for a very aggressive Purple Tang in the near future who will likely go postal on your other fish (especially the mystery wrasse, another fish known to have attitude).