Based on feedback I decided to go with the light indicator on the physical water meter because it is easier to understand (especially if you are having children help with watering), and because the specifics of the moisture level of the soil are in the app.
I had very mixed results in if people would prefer to buy a pack of water meters so each of their plants had one, or if they would want one in order to measure the moisture level of the soil. Among the people I interviewed it was 50-50. However, I decided to go with the multi-pack because it more specifically addressed the issue of remembering to water your plants. Having to go around to all of your plants and measure the moisture content of the soil adds another step, it does not make the task of watering more efficient.
I also added a button in the app to delete or get rid of a plant if it unfortunately dies. This is also paired with a reseat button on the back of the water meter. Other than that, not much changed in the app design.
Overall, I got very positive feedback on the design of the app and people felt as though this product would help them to be more consistent at watering their plants.