The brown on either side of the eye appears to go through the eye and is thus “the mask.” Like many frog species, he comes in different colors making ID less obvious as you can see in my Masked Tree Frog GALLERY or read about the Smilisca phaeota on Wikipedia. We found him in the fountain pool outside the Reception Lobby of Arenal Observatory Lodge.
And . . .
Historic Biodiversity Agreement in Montreal! Tico Times article