It depends on the type of club moss. They all tend to spread, but some more quickly than others. Selaginella plana really is not a ground cover and will completely take over if you let it. S. uncinata and S. kraussina 'aurea' have a slower growth rate and more of a ground cover appearance. In particular, S. kraussinia 'aurea' tends to form more compact, tight clumps (close to the substrate), while uncinata will spread, but also grow off the substrate to about 3-4".
Does that help any? I could try to post some pics if that might help.