Annoying people about my son's birthmark

My son has a birthmark on his hand. He was born with it. The Dr at the hospital said it was strawberry marks but the Dermatologist said it's a birth mark that might not go away. Today I went to the grocery store. A women putting out cheese said oh the baby looks sad because he has a rash. I gave her a dirty look. Then the cashier also said something. Like really why say anything...

It makes me sad to think of kids picking on him for this someday.