How do you say moss in Gaelic?

The Gaelic word for moss depends on which Gaelic language you mean. There isn't one single Gaelic language.

* Scottish Gaelic: The most common word is lus nan gleann (pronounced roughly loos nan glean). This literally translates to "herb of the glens" and is a more descriptive term. There may be other, more localized terms.

* Irish Gaelic: The word is cais (pronounced roughly kash).

Therefore, you need to specify which Gaelic you mean for a precise answer.