Getting mildew stains out?

Ashlyn • Aquarius mama to a Pieces and Virgo 💕 Instagram: ashlyngrvce
My favorite of my sons burp cloths is covered in mildew stains. 
I didn't notice it until I had already put it through the washer and dryer.. All I can figure is that someone threw a wet rag or something on top of it in the clothes hamper. 
I sprayed it with oxyclean and washed it again, but since it had already been dried it's pretty set in.
Anyone have any suggestions how I could get I out?