These changes are caused by drying (a.k.a. freezer burn) due to temperature fluctuations in the freezer and therefore the ice cream. Luckily, there is a way to prevent this issue.
Before closing your airtight container, place a layer of plastic wrap directly on its surface after each use, being careful to press it down to avoid air pockets. Homemade ice cream will keep in the freezer for about two weeks.