According to the stocklist it is a Chapa bug-eyed frog, Theloderma bicolor.
You did well, when I visited they were right in the corner of their tank, under an overhanging rock ledge
Yes, they are so hard to spot usually, I've spent many minutes watching a lump of moss waiting in vain for it to move! Thanks for the ID, I thought the display case said Mossy Frog, but it might be an alternative name or a misremembering on my part
