Any ideas what this could be?

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