Whom does Moses tell to "let my people go?" If the song is related to the slaves, whom might this figure represent?
In the context of the song, Moses is telling Pharaoh to "let my people go." Moses is speaking to evil Pharaoh. Moses is articulating the need for the Israelites who are suffering under Egyptian enslavement to be free. Moses is quite direct in his addressing Pharaoh. He is addressing him face to face and explicit about how evil Pharaoh must stop his ways of enslavement and "let my people go."
In the context of slavery, Moses can be seen as addressing the American slaveowner. Like Pharaoh, the American slaveowner represents the abuse of power in this life as he gains and profits from the suffering of those he enslaves. Similar to Pharaoh's overwhelming and all- encompassing Earthly powers, the American slaveowner controls everything: "Owners, overseers, traders, auctioneers—all function under his protection." It is for this reason that Moses speaks to the American slave owner, hoping to convince him of the need to stop his cruelty and "let my people go," in terms of freeing the slaves.