That is a tricky one. I gave up, and put a tiny green star sticker or marked on the atomizer that had menthol in it. You can mark on the connector with a green Sharpie pen if you have black attys.
I think it is best to just dedicate one atomizer to menthol.
You can try cleaning it out with Everclear alcohol. 24 hours, and blow it out, let it dry, and make sure to put lots of drops on the atomizer bridge.