I am building a raised bed on a lawn that is about 80% moss. I intend to put 6 inches of compost in the bed on top of cardboard. Do I need to treat or remove the moss beforehand?
Becky the moss is fine, will not affect your plants.
Only it suggests a shady and damp site which may (or may not) be more challenging, perhaps extra slugs. Whatever, I wish you good harvests.