Does giving parents mussar go against kabed es avicha vas ameicha?
While one needs to be careful any time one gives rebuke, one needs to be especially careful when dealing with a parent:
Kitzur Shulchan Aruch - 143:10
רָאָה לְאָבִיו שֶׁעוֹבֵר עַל דִבְרֵי-תוֹרָה, לֹא יֹאמַר לוֹ, עָבַרְתָּ עַל דִּבְרֵי תוֹרָה, אֶלָּא יֹאמַר לוֹ, אַבָּא, כְּתִיב בַּתּוֹרָה כָּךְ וְכָךְ, כְּאִלּוּ הוּא שׁוֹאֵל מִמֶּנוּ, וְלֹא כְּמַזְהִירוֹ, וְהָאָב יָבִין מֵעַצְמוֹ וְלֹא יִתְבַּיֵשׁ:
If one sees one's father transgressing a Torah mitzvah, one shouldn't say to him: ''You have transgressed a Torah mitzvah'' rather one should say to him: ''Father, isn't it written such and such in the Torah ?'' As if one is asking (for information) from him, and not as one warning him, and the father will understand (the hint) by himself and not be ashamed.
We see that one needs to have even more tact than usual when dealing with a parent.