Both frogs are probably stressed, but that's to be expected. Over the course of the last month or two they have probably gone from the wild, to an exporter in Madagascar, shipped to a large importer here in the States (or other country where you live), then to a smaller reptile dealer, possibly to a pet store and are now in your care. During that time the frogs probably haven't been kept in the best conditions so it can take a while for them to adjust to captivity. Keep a close eye on both of them and watch for any signs that would indicate that they aren't healthy.
For the first few days new arrivals often will often behave in strange ways. Sometimes they will be extremely bold and will hop around the cage all day, climbing to the top and then back down to the bottom. Other frogs, like the one you are concerned about, will simply hide all of the time only coming out occasionally to eat. Over the next few weeks they should settle down. From what I've observed with my painted mantellas (madagascariensis), they aren't extremely bold frogs and often hop back into hiding when their terrarium is approached. Food can lure them out into the open. Good luck,
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Devin Edmonds
devin@amphibiancare.com
www.amphibiancare.com