Yahweh asks this question to rebuke Miriam and Aaron. It can be translated as a statement. Alternate translation: "You should be afraid to speak against my servant, against Moses." (See: [[rc://en/ta/man/translate/figs-rquestion]])
The phrase "against Moses" clarifies that he is the "servant" of whom Yahweh speaks. Alternate translation: "against my servant, Moses" (See: [[rc://en/ta/man/translate/figs-parallelism]])