Jim, at the second go, was absolutely correct, they are nowadays referred to as 'commonplaces', although nothing wrong with 'cliche' when applied to a phrase. They include phrases, whole stanzas, and even in a few cases a whole series of stanzas (think 'rose & briar' ending in several ballads, and the sequence of stanzas in which the little foot boy runs off with a message). It's pretty certain that the 'rose & briar' ending was lifted from one of the earlier English ballads, but that's pretty irrelevant anyway as it exists in even earlier Danish ballads and probably others. 'The milk-white steed/hand/skin' can certainly be classed as cliché, but it's still a commonplace going back to manuscript era, so is found in the very earliest ballads. In fact I wouldn't be surprised to find it in pre-ballad material such as the romances. Someone with a knowledge of Arthurian lit will probably tell us it is.