What did slaves call harriet Tubman?

Answer 1

"Moses" is what she was called. This is a reference to a Gospel song about how Moses lead his people out of slavery to freedom. This is what Tubman was doing. Often various Gospel songs were used to relay messages among the slaves as they worked and the slave holder had no idea what they were doing.

Answer 2

She was called "Moses" because of a song that was used to relay messages about Moses leading his people to the promised land. Slaves often sang songs while working to give messages to each other without the slave holders understanding what they were doing.