Yellow patches on baby clothes that have been packed away for a while are almost always oxidation stains, not dirt. Sweat and milk residue left on a garment turns yellow or brownish as it sits in a drawer or box, even on a piece that looked clean when you folded it away. The longer it sits, the more set the stain becomes, so treat it as soon as you notice it rather than waiting it out.
Start with a cool water soak: mix in a few tablespoons of baking soda, or a cup of white vinegar, and leave the item for an hour before washing. For Epic's double-layer muslin, stick to the cold, gentle wash that keeps muslin soft rather than reaching for hot water or bleach. Chlorine bleach can weaken the weave and dull the dye, so skip it even on stubborn marks. Sun drying helps too: lay the piece flat or hang it outside, since UV light fades yellow stains on its own over a few hours. Tumble drying on low is fine afterward if you're short on time, but the sun does most of the real work.
Before you pack any outfit away, wash it on a cold cycle and let it dry completely, since any dampness or leftover residue is what causes yellowing in storage. Skip fabric softener here too: it coats the fibers and can trap the oils that turn yellow later. Muslin holds up well when it's cared for this way. Many parents find it lasts through more than one child if it's stored clean and dry, so a few minutes of prep now saves a stained outfit later. Store folded pieces somewhere dry rather than sealed in plastic, since fabric needs room to breathe even while it waits in a drawer.