Hi Mells...I know how you feel hey, and its so difficult as every1 will be coming with their own versions of what caused the mark.
My lg, now 8 years had it & every1 told me she has what they call "ibala" and had 2 b taken to a sangoma...I refused as I dont believe in them and read so much about it and found out that it is there because that is where the neck joins the head and the bones there are still soft, thats why the redness. ALL babies have this Black, Indian, Coloured or White, but its more visible on some and less visible on others. This made sense 2 me...and I followed my insticts and told myself it would go away and this I read can take up to 2 years, but of cos babies develop differently. It did go away after a year or so...
My 2nd lg has it too, and it doesnt trouble me as I now know what it is and know it will go away without any treatment.
As for the eczema, my lg has it and I just about tried everything till her Pead prescribed an Emulsifying Ointment to bath her with and Epizone A to moisturze, and her skin has never been so smooth. They both worked wonders
As a mother I say trust your instincts, and 9/10 they are right, so if something is not right or you are not totally convinced of the advise you get, you will know trust me.