What you use to remove will depend on what it is. If it is a hard water deposit, then an acid like vinegar will work. If you wash your jeep with hard water, and some water gets trapped in the hinge where it drains out, it will evaporate and leave deposits behind.
If it is a grease stain from lubricant on the hinge, than a caustic solution will need to be used. You could try a small amount of baking soda and water. A degreaser might work too.