**Benevolence is a matter of conduct not physical form.**

Ultimately if the aliens are benevolent (in the true dictionary sense of the word) their actions and the manner in which they introduce themselves and deal with us will reflect that.

Also if they truly are benevolent they must by default have at least some knowledge of humanity, its history and human society. This is because otherwise they wouldn't know how to explain or display their benevolence. (How do you know what some random person you meet might need needs until you get to know them and their background? )

So as other people have suggested robotic go between or just non-alarming digital images would do for a start for first contact during which they would explain that the images/machines are not their true form being quite upfront about that point. Then?

They just tell the truth and if they're worried humans will think they're horrible monsters, they start with some basic facts about biology and physiology, an explanation of life back home, images of it and then diagrams and still photos of their actual selves. (Breaking the news gently from a distance before face to face meetings occur.)