Will my baby’s mongolian spot go away?

Fades. The birthmark is harmless and usually fades between the ages of 3-5 years - a few will last up until puberty.
Usually. Mongolian spots, or slate grey patches, are a concentrated area of melanocytes (the cells that give your skin its pigment). Over time those cells spread out and the slate grey patch gets lighter as the baby's over all skin tone gets darker. Most slate grey patches are gone by the time the child is in preschool, though for some people they never completely disappear.
Maybe. I've seen some fade as the skin thickens and some persist well into young adulthood.
Yes. Mongolian spots are very common and can be mistaken for bruising. They usually fade by 8 years old. There is no current treatment to make them fade sooner.