A. serpens is among one of the smallest pleurocarpous mosses in Australia and is therefore easily overlooked. It grows on stone, soil or rotting wood in moist shady places where it forms low dull or yellowish turfs. The shoots are roughly pinnate, some of the branches less than half a millimeter wide.

The leaves are ovate to lance-like, tapering to a long point. A single nerve is present, reaching up to midleaf or higher but never to the apex.

The cylindrical and strongly curved capsules are rather conspicuous and are held on slender red stalks c. 1.5 cm tall.