So the MC of the novel our group is translating, Xiao Yue, is popularly known to the masses as this: 肖怀瑾 . We couldn't get an exact translation for this and this phrase is used a lot too. We'd go with Xiao Huaijin, ofc, but at least we'd like to provide the meaning of to the readers. Please help.
https://en.wikipedia.org/wiki/Courtesy_name if it is a historical setting then it is the courtesy name. "The courtesy name would replace a man's given name as he entered adulthood.[4] It could be given either by the parents or by a private teacher on the first day of school. Women might adopt a zi in place of their given name upon marriage.[4] One also may adopt a self-chosen courtesy name."
That seems very likely. For example, Zhuge Liang had the courtesy name of "Kongming". He'd be referred to in government documents as "Zhuge Liang", but in day to day conversations he'd be called "Kongming" most of the time. If this is the case, the character in the OP is probably spoken of as having a "名" of Yue, and a "字" of "怀瑾" somewhere in the text, usually as a form of introduction. If you see a word combination of "字怀瑾", then that's a dead giveaway so a Ctrl+F should be able to tell you for sure. If it doesn't show up then using "Huaijin" should be completely fine until the story gives you a different meaning.