Does the one on your arm stick up like it's sitting on top of the skin? If so, there's a name for those type moles and they almost never turn into anything.
I won't let my derm's P/A touch me. I figure I pay for a specialist and I don't want her guessing. She once did a biopsy on a mole that was mildly dysplastic nevus. Since he didn't know how much she got, he had to do the full excision to make sure he got clear margins, which meant a 3 inch wide, 3 inch long, 3 inch deep removal. It was 'all clear' but he said better safe than sorry. If he'd done the first one, he would have gone ahead and taken more in the first place.