"i'm doing something right now" => 나(는) 지금 뭐(를) 하고 있어요. => 나 지금 뭐(를) 하는 중이에요. 뭔가 (short for 무엇인가 = some unknown/undefined thing) is used when one is not sure what it is. In your example, the speaker is just choosing not to say what it is because it's not important. In such a situation, we just say 뭐 (short for 무엇, "what" in question context, "something" otherwise).

나 지금 뭔가하고있어요 나 지금 뭔가를 하고있어요 or with out 요 i don't know which one the most correct