First thing first, for the "无级／Wuji", I found an explanation from Baidu, which I think is quite helpful:
"Q: 无级温控旋钮是什么意思
A:比如，通过旋钮调节电压，调节范围是0到5V之间的任意值，这就叫无级。如果采用数字的，比如8位的，那么它只能取x/255的整倍数。这当然不是无级啦"
(source: https://zhidao.baidu.com/question/132959740.html)

Above example is about the knob of a temperature controller, and the knob is also Wuji type. And the "无级／Wuji" means the knob can be turned within a range (for above example, 0-5V, you can turn the knob to any position within the range), and the opposite example to 无级旋钮 would be the classic dial knob which has number indicated (if for the example above, the dial knob would have the scale maybe 1V, 2V 3V, 4V, 5V around the knob), and you can only select based on the number.

As for your question, it is a product description for humidifier, my opinion is, you may consider phrase like "adjustable mist control knob" or "fully adjustable mist output knob", you may also used the term "dial knob" if variable mist output is indicated around the knob.

Ps: you may also look through the online shopping website to get some other ideas

All the best :)