Girls, my son is 6 months old. A couple of months ago he had this kind…

Girls, my son is 6 months old. A couple of months ago he had this kind of episode once. I asked the pediatrician, she says it's nothing serious, but I don't understand what it means. Today I change his diaper and there's this spot, anyone who has experienced this, what do they say? I read on the internet that you should give more water, but my baby doesn't drink, he spits it out, sometimes manages to swallow it, but only in small amounts. (I apologize for the photo)