Why was this Parshah said to Moshe and Aharon (or to Moshe to tell Aharon)?
1.
Ramban and Moshav Zekenim: Because, since Kohanim issue all rulings concerning Tzara'as, it was not necessary to command Yisrael. Moreover, regarding Taharah from Tzara'as, Yisrael will willingly following the Kohanim's instructions, and the Kohanim will willingly bring the Korbanos.
1
The Torah writes concerning a Zav however "Daber el B'nei Yisrael", because it is a hidden matter that is unknown to others. It is therefore necessary to warn the [Zavim] to inform the Kohen.
