looks like a heat rash my son has one under his neck right now if its a heat rash put baby powder on it cause it will keep it dry and clear it up

heat rash.

Or it could be cradle cap my baby had it on her face and neck. try hydrocortisone that's what they gave my daughter when I took her to the Dr

It could be heat rash, their little necks are chubby and hold heat. if they have sensitive skin the rash will be more prominent and spread more. it could also be an allergic reaction to formula or if you're breastfeeding it could be something you ate. or even exema

looks like heat rash.