It could be the cause. However, Ficus can be very sensitive to chlorine and fluoride in the water. Chlorine is unstable and can be removed just by allowing the water to sit in an open container for 24 to 48 hours. Fluoride is more difficult to remove. I would suggest using either rainwater or distilled water for the plant.
I wouldn't depend on those glass globes, myself, but others might. Some people like the electronic moisture meters. I just go by the way the soil feels when I dig my fingers into it, or if the plant is small enough, I sometimes go by how heavy it feels when I lift it. I guess I'm old fashioned.
