While mostly when you see a moss like plant that is exactly what it is called: A MOSS! But not all moss looking plants are mosses!! The
most notable one about is Irish Moss, which is an Angiosperm or a flowering plant!
But if you have ever hiked about and seen some moss like plants growing on rocky hillsides in full sun then you just might be looking at a member of Selaginella of the subgroup (subgenus) called Tetragonostachys.
These are really great little plants. While it has been a while since I did extensive research on them I have not taken the opportunity until now to post up on the net some PDF field keys that I constructed for their identification. These keys are by state and are only for those states with a minimal; number of species.
If you are interested in using them check the link below to my thread on the Botany section that will link you to the keys.

Link at Botany Section