I'm sure one of the experts will chime in soon, but as I understand it, one of the life stages of ich can live on the shells, but not infect the critter. So, they can live in a tank that is fallow, as the ich will still die without a fish to infect during it's life cycle.
That being said, technically, any hard shelled invert, or coral, can transfer the one life stage of ich.
As I said, I am sure someone with more knowledge will name all those life cycles...