As the water evaporates and you continue to top it off, the water will become harder and harder since the minerals remain behind. Eventually that will most likely kill your shrimp, so I would recommend a regular water change to keep them happy. Also, you don't know which ferts are being used by your moss or to what degree. Unused ferts can eventually build up to toxic levels, which is why a typical fert routine includes a weekly 50% water change.
Better safe than sorry!
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.