I ran into something similar a while back, After almost a month in the simple green it did not budge. Used ultrasonic cleaner, Electric toothbrush and eventually a sculpting Pick to get some of the thicker areas down.
In my personal opinion when its thick and pervasive like that its going to be more work than its worth.
Others may have good suggestions but figured I would share mine. (Side note a month in simple green did not cause any issues with the plastic)