Why was this Parshah said to Moshe and Aharon (or to Moshe, that he will tell Aharon)?
Ramban, Moshav Zekenim: it is because Kohanim make all rulings about Tzara'as. There was no need to command Yisrael. Also regarding Taharah from Tzara'as, Yisrael will do so willingly, and Kohanim willingly offer the Korbanos. It says about Zav "Daber El Bnei Yisrael", because it is a hidden matter unknown to others; he warns [Zavim] to tell the Kohen.