Alcalcus baluensis is a small but stocky frog. Most specimens reach 21–30 mm snout-vent length. The tympanum is visible (not visible in A. rajae). The distributional range has yet to be clarified, but the species is known from Sabah and northeastern Sarawak. The altitudinal range is from the lowlands (200 m) to the highlands (1600 m).

The occurrence of this frog is spotty. It can be found in certain areas, but usually in small numbers. We found these frogs often associated with vertical rock faces at waterfalls and cascades, at times of low water levels in small forest streams. Most individuals preferred to sit vertically on rocks. The species may not be rare, but this is a frog not often observed by the non-expert naturalist, maybe because of its rather small size and good camouflage on rock surfaces. Little is know about its ecology. Details about its reproductive mode or larval development remain unknown.