https://blenderartists.org/t/simple-modeling-question-triangle-stays-when-moving-vertices/611072/6
@SonicBlue gives an important suggestion though, always prepare and upload an example .blend when you ask questions involving one. That move gives you answers instead of guesses, and those answers are more likely to contain visuals because you’re giving others a quick access to them.
If you can’t upload here, upload to http://pasteall.org/blend/ and share the download url.