I am growing a moss backyard but weeds are growing also. If I increase the acidity of the soil, (how best to do this) would it kill the weeds without damaging the existing moss?
No, increasing the acidity (lowering the pH) will not kill the weeds. Most weeds tolerate or even prefer a lower pH. The best way to control weeds will be by physically removing them or by misting the leaves with a liquid weed killer when it isn’t expected to rain for at least 24 hours. You can lower the pH by applying sulfur to the area but I wouldn’t do that. If the moss is doing well, it will create the level of acidity it prefers on its own over time.