Hi Kelley, and welcome to ECF!
Great that you're making the switch, your body will appreciate it
All toppers - the Nautilus, in your case - contain a coil / heating element, wick, and, of course, your juice. The coil/wick part, aka the head, is a replaceable part with a limited life. It will gunk up over time, and that amount of time will vary depending on flavor and a few other things. After a week, it may just be time to swap in a new one. (Some heads can be rebuilt, some toppers are designed to only be be rebuilt. But probably best to wait on that until you're more comfortable with what you have - and discover more about what you like

)
The discoloration is likely from some of the gunk becoming mixed in with the fresh juice. Unless it actually tastes bad / different it's not a problem, and is pretty normal.