The narrow light dorsilateral line and the rather pointed snout identify this odd little frog as Plethodontohyla notosticta. However, all of the accounts and photos of this frog that I've found describe its back as brownish or reddish, whereas this one is clearly greenish. So either the color varies more than the accounts mention, or this is some other species, perhaps an undescribed closely related one.

Here is a list of all the reptiles and frogs I saw on this trip to Madagascar.

This frog breeds in water-filled tree-holes, with the male hanging around apparently to guard the eggs and tadpoles. We could just see the snout and front feet of this one in a hole about six feet above the ground in a large tree trunk. Fortunately, even this little bit of frog is enough to identify it as Plethodontohyla notosticta, due to the angular snout. And there are only a couple of known species that breed in these tree-holes, which helps narrow down the identification.