umm.. we have very different version of "what's up" in Korean. You can say 1) Mo ha sam? (모하삼) : it's a more of kid version 2) Mo hai? (모해) : it's commonly used. Hope it helps,

안녕, 뭐해?"(an nyeong mwo hae) or "안녕, 뭐하고 있어?"(an nyeong mwo ha go iss eo)