Al Allaamah Ash Shaykh Al Uthaymeen was asked if speaking about the people of innovation is considered backbiting; he answered:
“Speaking about the people of innovation and those who carry unsound concepts or a methodology which is not upright, (then) this is from advice, and not backbiting.
Rather it is (an act of) sincerity to Allaah, His Book and His Messenger (by carrying out what is commanded and forbidding evil etc) and it is (act of sincerity) to the muslims (by giving them good advice).
So if we see an innovator spreading his innovation then it is obligatory that we make clear that he is an innovator so that the people are saved from his evil.
(Likewise) If we see an individual who has an ideology/concept opposing that which the Salaf were upon, then it is obligatory for us to clarify that so that people are not deceived by him.(Likewise) If we see a person who has a specific methodology (whose) outcome is evil then it is obligatory for us to clarify that so that the people are saved from his evil, and this (done) out of sincerity to Allaah, His Messenger and His Book; and it is sincerity to the rulers and their subjects (by giving good advice so that they saved from evil etc).
Regardless of whether speaking about the people of innovation is (done) amongst students (of knowledge) or in other gatherings, this is not backbiting.
And as long as we fear the spread of these innovations, or that concept or that methodology which is in opposition to the methodology of the Salaf; it is obligatory that we clarify so the people are not deceived by that.”
لقاء الباب المفتوح (120/8)