What is "Tzeva Hashem"?
Rashi: It is Yisrael, which is Hashem's legion. The angel was Micha'el, who is called "Sarchem" (Daniel 10:21).
Why must Yehoshua remove his shoe?
Alshich: It is due to the Shechinah, due to which "where you stand is Kodesh."
Metzudas David: It is proper to stand barefoot in a Kodesh place.
Malbim: Soon, Hashem's word will be revealed to you, so you must strip physical clothing from you, to be ready for prophecy.
Why was Yehoshua told to remove his shoe (singular)? Hashem told Moshe to remove his shoes (Shemos 3:5)!
Midrash Sechel Tov (Shemos 3:5): Moshe needed to remove both shoes, due to two things that he did not merit
Yehoshua did not have any sons (Tanchuma Shemini 3).
Ma'asei Hashem (Shemos 5): The Shechinah itself spoke with Moshe, so he needed greater humility. We are commanded to remove shoes on Yom Kipur for humility. Regarding Moshe, it does not say that he did so; he [also] hid his face, which is even a greater humility!
Why is the ground Kadosh?
Radak (14): It was according to the Kedushah and greatness of the angel.