"What's up" should be accepted. Reported.
'How's it going' is also a valid translation for 'Mitä kuulu', I believe
"Mitä kuulu?" uses a colloquial past tense form and means either "What was heard?" or "What did you hear?". "How's it going" is however an alternative translation for "Mitä kuuluu?".
Why is hello not accepted?
Hei is an informal way of greeting someone, but I think it's really over technical to mark hello wrong. Hello isn't much more formal than hi in my opinion.
Agreed. Reported. That's really all we can do...