Wa'alaikum assalam! Sayang, any unusual marking or indentation on baby's back needs to be checked by a doctor properly, jangan worry dulu, tapi kena confirm dengan paediatrician ye, especially at 6 months. They can examine your baby dan tell you exactly what it is dan if action needed. Best tak delay this one lah, book an appointment soon, okay? Mum friends I know, beberapa ada similar concerns, semua resolved after seeing the doc. You're doing the right thing by noticing it!