I dont have a White lipped tree frog but do you think he could just be changing colors to more of a brown depends on what the spots look like. Post pics? Usually if the frogs are active, eating and their eyes are clear i dont worry too much. But like i said i dont own a WLTF ....did some searching ... perhaps this will help ? frogs.org.au Community :: View topic - Leopard Spots on WLTF (with pics)
if not maybe you could check all the requirements for the frog and be sure they are being met.