Zygodon viridissimus is the commonest Zygodon species, but is virtually impossible to name with confidence in the field. Zygodon conoideus and Zygodon rupestris are also epiphytes that grow as very similar cushions, but there are some clues. A fruiting Zygodon is usually Z.conoideus which has slighter broader leaves in relation to the length, which tend not to be recurved, and when dry are more appressed. Zygodon rupestris looks much more like Z.viridissimus and tends to form yellow-green cushions on old trees, and both of these species have slightly twisted leaves when dry.
Z. viridissimus can also be found on limestone and stonework, especially damp brickwork. Zygodon stirtonii also tends to grow on stone but it has a strong excurrent nerve. Note that the leaves of Z.viridissimus end in an apical apiculate cell which can mimic a small excurrent nerve.
Under the microscope the gemmae have transverse as well as longitudinal cell walls, unlike the other species. This is the best character.Read the Field Guide account