In Britain and Ireland Bryum moravicum is almost exclusively found on trees, such as elder, ash and maple. As well as living bark it will also grow on rotting logs and on occasion crevices that have become filled with humus derived from leaves and twigs. The uniseriate gemmae in the leaf axils are common and distinctive.
The main problem is suspecting that it might not be Bryum capillare. This is a commoner epiphyte, looks very similar and is closely related. It tends to form tighter and denser cushions or patches than B.moravicum, which is generally a less bright green. Sometimes the patches of B.moravicum produce axillary gemmae in such abundance that they can be seen without a hand lens, but usually one has to hunt lower down the stem. B.capillare when dry has the non-decurrent leaves twisted around the stem, whereas the decurrent leaves of B.moravicum are shrunken and twisted but not arranged spirally. Beware of other Bryums with axillary gemmae from different habitats.
In practice the axillary gemmae are usually visible in the field but if they cannot be seen in an otherwise good candidate then check under a low power microscope. Be very wary of naming a plant as B.moravicum in the absesnce of gemmae.
Read the Field Guide account