Crusty tampon is that thing where your vaginal is so infected it boogers up and closes like an eyeball with pinkeye.
Usually there will be a tampon in there too, string hanging out.
Crusty tampon can also mean just a dirty old bloody tampon clinging to a bathroom ceiling for six months.
"Ooooh, my cooter is in distress, done got me a case of the ol crusty tampon."