Whites tree frog open mouth?

[LilyPad]

All you could do is get smaller crickets. Has he shut his mouth yet?

[DonLisk]

They shed routinely and one of the things they do while shedding is open their mouth really wide when trying to pull the shed into their mouth.

Could be hes shedding.

[UncleChester]

Was it repeatedly opening and closing its mouth or just sitting with it open? If the former, check the corners of the mouth for bunched up skin- it's probably shedding and this is fine. If the latter, well, that would be weird.