Why does the Torah add the prefix 'Kaf' in "k'Nega... "?
Rashi: Because even a Talmid-Chacham who knows for sure that it is Tzara'as, is not permitted to declare it such. Only the Kohen may do so.
Why does it say "Leimor"?
Hadar Zekenim (from Sifra 5:7): The Kohen tells him humbling words. Why does it say after "Hishamer b'Nega ha'Tzara'as" "remember what h did to Miryam"? Nega'im come for Leshon ha'Ra.