Would you consider this a birth mark?

Michelle • 26. Momma to Grayson 01.23.17 and Archer 11.21.18 💕

My baby is 12 months in a week and he’s had this on his back ever since he was born. I thought it was hair and first but now I’m not sure. It kinda looks like a bruise but it’s not. It doesn’t hurt him when I touch it and it hasn’t grown in side, but does tend to grow with him if that makes sense.