Moss on siding, do I need to replace this siding before painting

If it's real cedar, it shouldn't be more complicated than scraping, sanding, and restaining. You'll have to sand down to bare wood before restaining. Depending on what kind of siding you have, it might be hard to sand it everywhere, in which case painting it might be a better option. For paint, you just have to make sure you scrape/sand to the point where the paint has a good surface to adhere to. Then prime and paint.

But you should make sure you know what you have. Real wood is less and less common as siding, so you should make sure you don't have something like Hardiplank.

For my money and best looks, (if the wood is mostly sound) I would scrape and convert to an solid stain.. Won't peel, can be refreshed over old stain with minor prep (compared to paint)

Fiber cement siding would be my second choice, best durability, low maintenance, bit higher initial cost

Vinyl is not a personal favorite, would depend on what the neighborhood had.