I have a brown spot in my left eye. will it go away or should i see the doctor?

Best to see your doc: Assuming this brown spot is on the cornea, it could be a stain or rust-ring from a metallic foreign body. See your ophthalmologist to give you a definite diagnosis and treatment plan.

Pigment in the eye: You must see an opthalmologist to make sure it is a beniign coloration rather than a concerning pigmentation that needs urgent attention.