My son had that only when he laid on his back to long. Then it would disappear

My son has had the same since he was born and he's 7 now. It's a birth mark type thing I believe. (That's if it's the same)

looks like a little bit of a heat rash...my daughter got it there a lot and it would fade...she doesn't get it anymore

Yes same with my son @tibi2696

My son has had that since he was born I just figured its from moisture getting trapped there or something it never bothers him and fades sometimes