The closest translation for "shwmae" would be the slang term "alright?", wouldn't it? "alright?" is used as a greeting meaning of "Hi" or "how are you?". That's how it's been described in South Wales as far as I've experienced.
What? No, not S'my, S'mae!
But what's yours?
You're killing me, Smalls...
Not sure if this should be accepted or not, but in some North American dialects, "How is?" is used as a slangy greeting. "S'mae" is a contraction of "Sut mae" - literally "How is?" Should we accept "How is?" as a translation for it?
Because although you are right, 'shwmae' basically means 'what's up', Duolingo would use a simplified version of the literal translation.
"hello" is accepted in one response, but not accepted here. Suddenly the answer wanted seems to be in American. This is not a criticism because the site is American, but it is odd that a correct response should be marked wrong later.