Dorsal coloration is uniformly light to dark brown or blackish,[2][4] without any specific markings.

There are few, scattered white spots on the sides of body and head, the lower parts of the flanks, as well as on the hands and limbs.

[2] Alcalus rajae occurs in primary undisturbed and secondary forests near seepage areas close to small and slow-moving streams and swamps.

In contrast, most of the original vegetation in the Gunung Penrissen Nature Reserve has been lost because of slash-and-burn activities and tourism and recreational development.

Outside the known sites, the species would be likely suffering from deforestation caused by oil palm plantations and forestry concessions.