במדבר פרק יג, טז א לֶה שְמוֹת הָּאֲנָּשִּים אֲשֶר-שָּלַח משֶה לָּתוּר אֶת- הֶשמ אָּרְק יַו ץֶרָּאָּה לְהוֹש עַ בִּן-נוּן יְהוֹשֻׁעַ:
Rashi explains here, based on the midrash, that Moshe's intention in changing his name was to add a 'yud' to the 'heh' so that it would spell G-d's name, as if to say 'G-d should save you from the plot of the spies' (because Moshe understood that it was possible that the spies would sin, and he prayed that it shouldn't happen to Yehoshua). We have to explain why Moshe prayed for Yehoshua to be saved from speaking lashon hara about the land of Israel more than for he prayed for all the other spies.
Perhaps we can explain based on the Medrash Rabba in Parshas VaYeshev (end of section 86) regarding the nature of people. 'Throw a stick in the air and it will return to where it came from”. This is an analogy to people who inherit their behavior patterns from their parents. Like we find in the Medrash Rabba, Parshas Miketz, that when the goblet was found in the sack of Binyamin, and the brothers thought that he had stolen it, they all called him 'thief, the son of a thief'. They thought he was following in the footsteps of his mother who had stolen the idols from her father (Vayetze 31, 32). We also find this as a common saying amongst people, 'the lamb follows the ewe - the actions of the daughter are like the actions of the mother' (Ketubos 63a). Also in Yechezkel (16, 44) we find “the daughter is like the mother”.
We know that Yehoshua was from the tribe of Ephraim ben Yosef. Yosef had the attribute of speaking badly about others, like we find at the beginning of Parshas Vayeshev (37; 2) “Yosef brought evil reports (about his brothers) to his father”. Therefore, Moshe was concerned that this attribute might be part of Yehoshua's genetic makeup. Since he was so close to him as his student, Moshe prayed for him in particular, which explains why he didn't also pray for Gadi ben Sussi from the tribe of Menashe, who was also descended from Yosef.
We gave a similar explanation to something which we find many times in the Gemara. When Rav Yosef was amazed about something he would say 'Master of Avrohom' (for example look in Shabbos 22a, and the other places listed there). We don't find anyone else who used this expression, nor is it explained why Rav Yosef chose this phrase to express his surprise or amazement, or why it was unique to him.
Perhaps we can explain based on what the Ran brings in Kiddushin (31a) in the name of an aggadah. Rav Yosef was careful not to look outside of his immediate four amos. For this purpose, he damaged his eyes [which eventually led to him becoming blind]. This trait of never looking outside the four amos was also a trait of Avrohom. Rashi explains similarly in Parshas Lech Lecha on the verse “Now I know that you are beautiful” [that until that moment Avrohom had not noticed Sarah’s, his wife, beauty]. Also, in the Midrash Rabba on Vayera (Parshas 53) they said that he fulfilled the verse “close his eyes from seeing evil” (Yishiyahu 33, 15). Because of this trait of not looking at things not near himself, Avrohom didn't notice the actions of Yishmael. Therefore, Rav Yosef had the same trait as Avrohom, and therefore used this expression of amazement 'Master of Avrohom'.
We also explained similarly the Gemara (Chagiga 14b), which tells of Rabbi Elazar ben Aroch who expounded upon kabalistic secrets of the nature of G-d. After he finished, Rabban Yochanan ben Zakkai stood up and kissed him on his head, and said, 'Blessed is G-d who gave such a son to Avrohom our father'. He meant this same idea, because we know from Medrashim that Avrohom explored these kinds of ideas, and as explained by Rambam at the beginning of his laws of idolatry. Therefore, he connected Elazar ben Aroch to Avrohom because he followed in his footsteps.
