they sell some nice re-creations of marine invertebrates now days, and they seem soft and rubbery, so I don't know if coraline would be a problem on them or not. but when you get right down to it, coraline, or nuisance algae is going to be an issue for him anyhow, depending on how well he cares for the tank. how about instead of using dead sps's or fakes, you opt for some nice macro algaes? there are some gorgeous red ones out there, and they would actually contribute to the tanks health.