Grudge Off is a soak based reusable glass cleaner.  This stuff is non abrasive and doesn't feature any cleaning crystals (epsom salt). Fill up small container and place your pipe in it.  Let sit overnight strain any chunks of particulate through a mesh screen and place back inside the bottle.  If there just that one piece you have never been able to get clean.  Grundge Off is your best bet and Nhalables is your convenient place to snag it.