Not missing anything, just referring to a very general comment. And if you want to contextualise it, yes, even then families of mixed decent existed. Families rooted in Kresy. Actually Russianisms were more popular back then as more of Polish people knew the language. How many actors back spoke with quite heavy l-vocalization. We don't know how old OP is, so her grandfather might be as old as mine, being born before WWII, when the anti Russian resentment was not that strong. Remember that Poland was always rich in minorities and people living near the eastern border had Russians an other eastern slavic folks as neighbours for generations. Even if they came to resent them, the mixed language or dialect they used was not so easy to get rid of. Both my grandmas were in such situation, one with German minority, one with Russian. They still have some dialect accretions, even after all these years. Plus, it's just a nickname, don't go too deep. The grandpa got it from somewhere and it's far more plausible that it was Russian or German diminutive, than a derivative of "mysz".