Is that normal?

So I am 17.

My mum married a guy few years ago who isn’t

My dad.

And for a while now every time I walk past him he’ll slap My ass and I don’t know why. It makes me uncomfortable and I’ve tried saying stop but he keeps doing it and even when he walks past and he doesn’t do it I flinch. and my mum doesn’t seem to notice or care

Is that normal? Or should I be worried

Is my mum love blind?