The Grocestapo is the elderly gentleman in the grocery store who glares at you if you speak too loudly, or angrily scolds kids who run. To him, a grocery store has many unwritten rules of conduct, and he plans to enforce them... even the ones only he knows... which is most of them... or all of them.
"You have to buy any loaf of bread you touch," an old man once tried to teach me when I decided I'd rather go with the sourdough. I didn't listen to him, because the Grocestapo has no authoritative power. He's just an angry, old jerk.
After that many years of people running into you with grocery carts, and seeing your favorite brands and products discontinued, I suppose it's no wonder there's a Grocestapo in every grocery store I've ever been to.
Should you be wearing slippers in a grocery store, Grocestapo? Seems like you'd be on top of something like that.