HG Schild asks:
Why Moshe HaTzadik in Mishna and usual Rabbenu in gemara? Why lashon HaTZadik here and where else why?
hg
The Kollel replies:
Shalom Rav,
I think the Tana refers to Moshe as "Moshe ha'Tzadik" in order to stress the importance of Milah in that even though Moshe was perfectly righteous, it did not save him from being punished for not performing the Milah straightaway.
B'Virchas Kol Tuv,
Eleizer Chrysler