Let's break it down:
* Que: This is a contraction of "qué," meaning "what."
* estas: This is the second person singular present tense of the verb "estar," meaning "to be" in the present continuous tense.
* haciendo: This is the gerund form of the verb "hacer," meaning "to do."
* mija: This is a term of endearment for a young woman, similar to "honey," "dear," or "sweetheart."
So, the phrase is a casual question asked to a young woman to inquire about what she is doing.