It's plastic, and it's gross. I was using tea tree oil in the water, but then didn't use it for awhile. Kept forgetting to empty it, and when I did, it was slimy. No visible mildew/mold, but it was slimy. I washed it out as best I could, then poured vinegar in to dissolve the slime. Now it looks like I may have damaged the plastic, or else the slime dried and needs to come off. It's a mess.
What do I do now? Run it with vinegar, like a coffee maker? Something else? Is it totaled?