In my shiur this morning, I pointed out that although Rav Moshe has a teshuva explaining that ma'aris ayin only applies to situations where one is doing something that is mutar, but other people think he is doing something else which is actually assur. For example, one who hangs out soaking laundry to dry on shabbos, and people think he laundered them on shabbos, that is ma'aris ayin. But if one were to put out their laundry to dry and people will think he violated shabbos because of their mistake in thinking that drying laundry is also prohibited on shabbos, that is not considered ma'aris ayin. Nevertheless, it seems from our gemara that even if one is doing something that is mutar and other people will mistakenly think it is assur, an adam choshuv should refrain from doing it as rashi explains - שהרואה אותו שהוא מיקל עומד ומיקל יותר. In the case Rav Moshe was describing the sho'el was giving him mussar for driving to shul on friday afternoon after candle lighting since some people think it's assur. After rejecting the problem, Rav Moshe agreed to stop this practice but didn't explain why he agreed. Perhaps he is relying on this gemara.
I later realized that I had written about this about 5 years ago: