Neither a moss or a grass, Sagina is from the same plant family as dianthus (pinks and carnations) and good number of other groundcovers. It will not grow in the same conditions preferred by a true moss - it wants sun, good drainage but even moisture and a pretty fertile soil. If you provide these conditions, it should do very well but I find it is not the most rapid of spreaders and will have a tendency to mound as well.