I would think it's like this- if mzima means whole, complete and healthy, it's kind of like the extremely casual English "you good?", except it's a common greeting and not as casual. "Are you well?" is also a comparative greeting, though it's not used much and implies sickness.
To the best of my knowledge, hujambo literally means "you have no troubles?" and means "are you well?" But they are used in the same context. And in fact, greetings can be such a long process, in parts of Tanzania at least, that both might be used in the same conversion.