I haven't encountered this before. And I use RM2's over 90% of the time for several years now. Did you
buy this from the website ? or from the classifieds ?
I would try about a 5- 7 minute soak in 91% Isopropanol, followed by a 5- 7 soak in boiling water. I vape a lot of NET's and sweets, and my post/ screws/ ceramic decks will turn dark after awhile. This method of cleaning seems to clean it up, then I wipe everything with a cotton swab. Seems to work for me. Maybe the color you're seeing is just early build up from the custard you vape.