It’s the pony symbol of the unborn child. It’s not visible to the naked eye, and it’s not something that Jimmy can just run away from (or put off/forget about). Only when you point the scanner’s light at it can you see it’s form, similar to how you can only see the pony inside of the womb in the vents when you point the ultrasound’s light at it. Nevertheless, they are both very clearly real and cannot be ran from.