That is moss. It could be mounded up...it could also have developed that contour over time.

You could get a similar effect from baby tears, Soleirolia soleirolii, if sufficiently moist. Or Scotch or Irish moss, Sagina subulata, which is not a moss at all, prefers a sunny aspect and tends to mound all on its own.

I believe that is just moss- check out Moss Acres dot com for some amazing mosses for sun or shade