Good question!
To answer the first part of your question – moss tends to grow on the shady side of trees, rocks, and other objects because it helps them to retain water (moss don’t have a water transport system of veins like bigger plants, so they need to be in damp conditions). Moss often grows on the north side of trees (in the northern hemisphere) because south facing things get more sun (due to the tilt of the Earth). In the southern hemisphere, the opposite is true – the north side of things gets more sun so moss would preferentially grow on the south side there. Regarding the second part of your question, from my understanding, slaves used a variety of naturally occurring signals, which included both the orientation of moss and navigation by stars (among other things), to guide them in their journey North.