I used have a IR soldering station, for soldering BGA chipsets. The preheater on the bottom was ceramic & got stained from rosin flux. I never was able to clean the stains but that was flux. Didn't affect anything.
juice might be different but then again it could be similar.
Have you tried rinsing with hot water or boiling the ceramic insulator only? Soak in isopropyl? I clean my atties in a heated ultrasonic cleaner & they come out squeaky clean. I don't have a stained RM2 to try though.