How long have the rocks been there in their current condition? Unless that habitat only recently became available, I'd guess that the mosses have filled the hospitable bits of the rock. If that's the case, the only way to get the moss to expand is to increase the amount of rock that's habitable to moss... this would generally mean decreasing sunlight, increasing moisture, or both.

Patrick Alexander

I've seen moss form little gardens around water spigots in full sun in Florida. The constant moist/wet conditions allow the moss to grow even in the sun. So Patrick's correct, you have to extend those conditions over the area you want to be moss covered. Unlike seeds, moss spores germinate and then the gametes have to swim to join and form a new moss plant. To wit, no water to swim in and no new moss! PF