The second sign at the burning bush is more disturbing than the first. The serpent was outside Moses' body; the leprosy is on it. Targum Pseudo-Jonathan preserves the bluntness of the scene: Moses puts his hand inside his cloak — the Choba, in the Aramaic — and when he withdraws it, his hand was leprous, it was white as snow.
In the biblical imagination, tzaraat — translated loosely as leprosy — is not primarily a medical condition. It is the outward manifestation of inward corruption: slander, arrogance, misuse of speech. Leviticus 13 devotes an entire chapter to it. The Talmud (Arakhin 16a) will later list seven sins that bring it on. When Moses' hand turns white as snow, the sign is not random.
What Moses Is Being Shown
The sages of the Targumic tradition read this as a warning addressed to Moses himself. Moments earlier, he had said of the Israelites: they will not believe me (Exodus 4:1). He had spoken suspicion against his own people. The leprosy flashes onto his hand as a mirror: this is what happens when a leader slanders the community he is being sent to serve.
The takeaway: the second sign is a prophecy about Moses' own danger. The prophet who carries a staff of sapphire must also learn to guard the hand that holds it. The leprosy is white, pure, terrible — and reversible, as the next verse will show.