A false apostle is one who pretends to be a teacher and witness of true doctrine

Chapter 42: 2 Corinthians 6–13
Elder Bruce R. McConkie described some differences between true and false apostles in our day: “In the general sense, a true apostle is an especial witness of the Lord’s name, one who knows by revelation that Jesus is the Lord. A false apostle is one who pretends to be a teacher and witness of true doctrine without having the requisite personal revelation. In the specific sense, a true apostle is one who has been ordained to that office in the Melchizedek Priesthood and who normally serves as a member of the Council of the Twelve, and who therefore has power and authority to govern the Church. A false apostle is one who professes to have power to govern the affairs of the Church on earth, but does not in fact have the requisite endowment of divine authority” (Doctrinal New Testament Commentary, 2:440).
https://www.churchofjesuschrist.org/study/manual/new-testament-student-manual/2-corinthians/chapter-42-2-corinthians-6-13?id=p49#p49