I had a chat with a guy at the pet shop about moss today. He says the green moss will grow and the gray moss won't.

I have a rather large bonsai collection and I also get moss. You need to find some and place it onto the surface you want it to grow on to. Maybe try tying it with a elastic or something, then all you need to do is to keep it moist. even if moss dries out completely u can take it, sprinkle it over the area you want and start to water by keeping it moist and it should grow again. also keep in mind that there is different types of moss

I have read that in order to get moss to spread is to take plain yogurt and smear it over the surface you want the moss to grow on. Apparently it really works.