This a normal problem. If the fridge doesn't have a defrost cycle like stated above you will have some ice on the fins especially if you are in an area with higher humidity. Every time you open the door moisture laden air enters the fridge and it will collect on the coldest thing in the fridge which happens to be the fins.
When we lived in out 5th wheel for a project in Oregon we had to manually defrost the fridge a few times. Our door seals where good and there was no kids grazing in the fridge to keep it open for long periods of time. We also had a bar style fridge in the basement to precool items and keep our adult beverages in. This kept me out of the regular fridge to get a beer.