The Hand of Musa turning White without any Disease

This is the second sign of Musa. That is Allah has commanded him to place his hand into the opening of his garment, as is clearly stated in another Ayah. It mentioned here merely as a passing reference, saying:

وَاضْمُمْ يَدَكَ إِلَى جَنَاحِكَ

وَأَشْرِ‌كْهُ فِي أَمْرِ‌ي (and make him share my task - 20:32): Sayyidna Musa (علیہ السلام) had the power to appoint Sayyidna Harun (علیہ السلام) as his Wazir on his own, but he petitioned to Allah Ta` ala to confer the appointment on his brother in order to seek His grace. Besides, he wanted him to share his prophethood and his mission and this power does not rest in any Prophet. Therefore he specially prayed to Allah Ta` ala to make him a partner in his mission.