Thanks for all the answers and help.

At the moment, I still have the green line indication, but I think I know the reason of it. At the beginning, I thought it was a formatting issue, as there are first letter capitalized word in the middle of both paragraphs, and paragraph 1 doesn't have the green line. It makes me think that WORD might have a setting that allow the situation in paragraph 2, under the normal grammar & formatting checking rule. Because even if I turned off (or 'ignore') the checking indication, other people will still see it when they open the doc in their word.

Later of that, I tried different words at the highlighted area, and only some word combination didn't give the 'green line'. So I think it's a grammar issue, and there is no way to top WORD give the green line indication for this situation unless turn the checking off or 'ignore' it.