Why did Hafs, citing Asim, recite: (قَالَ رَبِّ احْكُمْ - He said: My Lord, judge), while others recited: "قُلْ رَبِّ احْكُمْ - Say: My Lord, judge)" in verse 112 of Surah Al-Anbiya, and what is the difference between the two recitations?

The difference between Hafs's recitation of "Qala Rabbi" (He said, "My Lord") and others is that Hafs's narration is an account of what the Prophet (peace be upon him) said, and therefore the verb is in the past tense. As for the recitation of others, it is in the imperative form, where Allah commanded His Prophet to say, "My Lord, judge with truth," just as the prophets used to say, "Our Lord, decide between us and our people in truth."