I would be very wary of using ordinary bleach on your muslin, it can cause yellowing and actually rot the fabric

especially delicate fabrics.
If I have this type of problem I use Dispel OXYGEN fabric stain remover. It can be bought cheaply in tesco and can be sprayed on or soaked, it can also be used in various strengths. It doesn't leave a smell and hasn't damaged any of my delicates yet :D