Take said Pokemon to the move relearner with a Heart Scale and relearn the Lv1 move.I am embarrassed to say that through all my research I have yet to come across an acceptable explanation of what a Lv1 move is and how to acquire it.
You can't, well not without trading (Mothim is impossible to trade to FR/LG).I don't remember being able to catch Mothim in FR/LG. (Or Beautifly, Masquerain, or "Bustox" for that matter...)