My best guess would be you are keeping him/here way to dry.

Seeing the substrate and moss, the humidity seems rather low.
This seems to be confirmed by you're frog hiding underneath the moss,
this may mean two things:

1. Underneath the moss it is a little more moist, so he seeks refugee there
2. The temperature is lower under the moss sheet, altough they might withstand 30 degrees celcius, so my guess will be it is because of the humidity.

My guess would be he is very inactive as well.
Try raising the humidity by spraying and watch if he/she will get more active.
If you get the frog active again, he/she might start eating again.

Is the frog in hunger strike for long? Maybe it is needed to let it hydrate,
i believe you guys use electrolite with the bathing for that purpose,
but you can better ask someone who knows good brands around you;re country for that purpose when needed