Everything in @Paribus's answer is accurate, and most of it is even mentioned in Yunmai's own quick start guide.
How does bioelectric impedance analysis (BIA) work?
In BIA, two or more conductors are attached to a person, and a small
electric current is sent through the body. The resistance between the
conductors will provide a measure of body fat because the resistance
to electricity varies between adipose, muscular, and skeletal tissues.
... Body composition analysis is an inexact science. Body-fat scales are rarely 100% accurate compared to professional and precise medical instruments. This is true regardless of the manufacturer or method of measurement.
It took a bit of digging to find what their "protein" measurement actually meant, but you can see it in their light2 specs.
Proteins are the main building block of the body. They are used to make muscles, tendons, organs, and skin. The percentage of protein in total body weight is around 18%.
Basically they are simply trying to estimate what percentage of your weight is from proteins. You mention that 16-20% is in the green and that you are at 16.9% which is in the green range. Even if you weren't, I wouldn't really worry about that measurement even if it was perfectly accurate. If you are worried about low protein levels anyways, eating more protein and staying fit is can't hurt.