The white moss (Leucobryum glaucum)? Surely it is green? You actually need to see this moss close up to see why it is called white moss. The shoots are actually different colours, some are a dirty white, some a glaucous colour (see the glaucum in the scientific name) and some are pale green. From a distance it looks green but close up it looks more white.
This is a common moss in acid conditions, especially on damp soils in woods, heaths and bogs; the soil must be bare and very acidic. It forms tufted mats of closely packed shoots and it is sometimes called the pin cushion moss for this very reason. In favourable conditions these mats can be quite large and cover earth, logs, stumps and even tree trunks. It is very distinctive and cannot really be mistaken for any other moss species which, in moss identification, is a real treat for the novice!.