I understand that idiomatically this translates to 'what is your mother's name', that's how we say it in english. But the literal translation is 'how is your mother called?' That should also be accepted. 'What is your mother's name' would be 'Cual es el nombre de su madre?'
Your mother's name, what is it?
The name of your mother, what is that.?
The name your mother has, what is that name?.
The name, your mother's?
The name of your mother?
Your mother, what's her name?
Your mother, name?
This is a list of alterate translations that Duolingo does not accept to go along with all those other people are providing above and are complaining about how they are not accepted. My list is just as legitimate. Bet I can come up with more.