It would be nice if there was live rock for fresh water, but it just ain't so. We use plants as a substitute. Plants will help keep nitrates low, some better than others. Some people even dose added nitrates to the tank because the plants suck up too much of them.
Mos isn't one of the plants that help much in this department. It grows very slowly and doesn't use much nutrients at all. You want fast growing plants for this purpose, and those are normally ones that need lots of lights. There are a few like hornwort (one of my favs for this) that tend to be nitrate sponges, sucking up nitrates like mad. I even keep a bunch of it in my in-tank refugeium (sort of a tank within a tank) in my 90 to keep nitrates as low as possible. I keep it in this to keep the severums from eating it.