Why does the Torah insert the word "me'Cheiko" here, but omit it in the preceding Pasuk (4:6)?
Rashi: To teach us that Hashem's Midah of goodness works faster than His Midah of punishment. 1
Rashi: Because "me'Cheiko" implies that even while his hand was still in his bosom, the skin had reverted back to normal; whereas in 4:6, it only turned into Tzara'as after he had removed his hand from his bosom.
QUESTIONS ON RASHI
Rashi writes: "'It had [already] turned back' - We see that [Hashem's] goodness comes more quickly than [His] punishment." Why is this so?
Gur Aryeh: Existence is synonymous with goodness. Hashem desires existence, not harm or destruction. 1
Maharal (Chidushei Agados Vol. 1, p. 46, to Shabbos 97a): Hashem is good, and abundant in Chesed (Tehilim 86:5). All is arranged such that good should come from Him always. Harm, i.e. withholding of good, runs against this.
Perhaps this is along the lines of Ramban (to Bereishis 1:10) - "Ki Tov" means that Hashem desired that this item exist, in this form. (EK)
Rashi writes: "We see that [Hashem's] goodness comes more quickly... for in verse 4:6 it did not write 'from his bosom'" (whereas here, it does). Conceptually, why did Moshe's hand not contract Tzara'as until he brought it out?
Maharal (Gevuros Hashem Ch. 27, p. 111): When Hashem instructed Moshe to place his hand in his bosom, it became like a "Beis ha'Sesarim" which is not susceptible to contracting Tum'ah. 1 It became Tamei only at the moment he brought it out.
Maharal (ibid.): In the conceptual sense, Tum'ah comes only from Kochos Chitzonim, whereas Taharah comes from a lofty, concealed place. This corresponds to what Rashi writes (that Midas ha'Tov comes more quickly).