I've heard it looks like snot with blood in it
The plug is a jelly like substance which sits at the end of your cervix, sealing your uterus off from infections etc. It can be lost hours, days or weeks prior to labour, although losing it before 37 weeks you need to inform your midwife. when you lose it, it can be one big white gloopy mess, be green and snot like, or be small white jelly like pieces, once your near labour, it can present as a bloody show. Hope that helps x