This is not the border. It is how the native EditText control looks. It is best to probably keep it. You can cast it to EditText and change its Background property.
Thanks An Schi, but in B4XFloatTextField as a password and reveal set, when you ae editing and press the "reveal" button it loose the outside configuration.
But using your ideia you can set de background color as a transparent in the designer and the line just appears when you are editing the field.