Based on the level of respect or formality, you can say "What are you doing?" in Korean in 3 ways:

• Informal: neo mwo-hae? (너 뭐해?), neo mwo-hani? (너 뭐하니?)
• Formal: dangsin-eun mwo-haeyo? (당신은 뭐해요?), dangsin-eun mueot-eul hapnigga? (당신은 무엇을 합니까?)
• Very Formal: dansin-ggeseo-neun mueot-eul hasipnigga? (당신께서는 무엇을 하십니까?)

- what (objective): mueot-eul (무엇을), mwol (무얼), mwo (뭐)
- you (subjective): neo-neun (너는), dangsin-eun (당신은), dansin-ggeseo-neun (당신께서는), ne-ga (네가), ni-ga (니가)
- to do: hada (하다)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.