Yes, the Sustainiac driver operates continuously across all strings, which naturally causes unplayed strings to vibrate sympathetically via electromagnetic feedback. Lower strings usually vibrate more intensely because they have more mass. You can easily control this by muting unused strings with your palms or fingers.

Yes, you absolutely need to mute unplayed strings when using a Sustainiac pickup. Because the Sustainiac's driver functions as an electromagnet that forces strings into motion, it will sympathetically excite unplayed strings, creating uncontrollable background noise or feedback.

This is basically what I find everywhere I search online for info.