Moshe does not enact the third plague. Rashi explains that,
לֹא הָיָה הֶעָפָר כְּדַאי לִלְקוֹת עַ"יְ מֹשֶׁה, לְפִי שֶׁהֵגֵן עָלָיו כְּשֶׁהָרַג אֶת הַמִּצְרִי וַיִּטְמְנֵהוּ בַּחוֹל,
That the dirt protected Moshe when he killed the Egyptian and buried him in the sand.
This comes from Sh'mot Rabbah 10:7
אָמַר רַבִּי תַּנְחוּם אָמַר לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא לְמשֶׁה עָפָר שֶׁהֵגֵן עָלֶיךָ כְּשֶׁהָרַגְתָּ אֶת הַמִּצְרִי אֵינוֹ דִין שֶׁיִּלְקֶה עַל יָדֶךָ
My question is -- in what way did the sand actually protect Moshe? While, yes, the body was buried, how does this protect Moshe? He had already been spotted and because of the comments of the other Hebrew men, he was in trouble and had to run away, regardless of the sand. What, exactly, did the sand do to protect him that it deserved such consideration?