I put "What would you like to say about that?" and it was accepted. It's better English, and invites someone to comment & say more about something, but it doesn't mean quite the same as "What do you mean by that?", which asks for a clarification or explanation. I can see the politeness in using "moechten" instead of "wollen", but what is the importance of using "aussagen"here?
@FisherLiz: IMO it shouldn't have been accepted. "What do you mean by that?" is a better translation. As to the use of 'aussagen': It's certainly possible to use it here, but I'd prefer 'sagen'. It would be fine to say "Was wollen Sie damit sagen?". It's not considered impolite.