hi..thanks for your query..
The patches of the arms may be a result of any injury/ inflammation/skin disorder that might have occurred in the past.You should be able to elicit a history in such cases.
The next possibility is of a naevus, which we commonly call a birth mark..there are several types of the same.There may be associated patches elsewhere in the body as well.
The only way to know the exact cause is to visit a dermatologist
who can guide you by looking at the morphology of the patch.Also, a definitive diagnosis an be attained by doing a punch biopsy
of a part of the patch.
Hope this helps.take care!