Må doesn’t have a direct English equivalent, but it means ’to feel, to be in a certain physical or emotional state’ as in:
- Jag har varit sjuk, men nu mår jag bra. (I have been sick, but now I feel fine.)
- Hur mår du? (How are you? [i.e. sick/ill/healthy etc.])
Hur är du? would not refer to your emotional or physical state, it would simply mean ’how are you?’ as in perhaps ’how are you as a person?’ (how would you describe yourself?) and you cannot use it when talking about feeling fine or being ill etc.
You shouldn’t use hur är du? It is not used as a greeting, as I wrote above. However, it’s common to say hur är det? (how is it, i.e. ’how are things?’).
The difference between hur är det? and hur mår du? is that hur mår du is used when you are genuinely curious about the other person’s health.