Changing mattress for Ikea changing table

Changing mattress for the popular Ikea dresser. Ordered from Little Loves.
Beautiful, quality, soft, served one child for a few months.
The child was never changed on the mattress-always put a waterproof sheet. Can be washed at 30 degrees.
The work surface is in perfect condition, I don't see any stains, but on the other side there is a faintly noticeable stain, I don't know what it is, but there is no stain on the top. Visible in the last picture.