Alcalcus baluensis is a small but stocky frog. Most specimens reach 21–25 mm snout-vent length.

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 and cascades at times of low water levels in small forest streams. Most individuals preferred to sit vertically on the rocks. It may not be rare, but this is a frog not often encountered by the non-expert naturalist. Little is know about its ecology. Details about its reproductive mode remain unknown. 

It differs from the other known species from Borneo (I. rajae) by its smaller size and visible tympanic ring.