If you haven't already removed all the moss, you should go ahead and do so. If it's there, they'll find some way to eat it.

Other than that, your husbandry seems spot on, so you may be looking at an impaction. Either way, the treatment for impaction doesn't harm them.
Feel the frog's right side gently, if you can feel a lump that may be an impaction.

Fill a container with treated water up to the frog's chin-level, and mix in 5-7 drops of honey. Let the frog soak for 10-20 minutes. You can massage the lump gently to help the frog pass it. You can do this once a day until the frog passes it.