I agree with
Robert, it does look like malachite, but if you are
still unsure, just leave the coin in the open air for a about two weeks. If it is BD, you will see it start to grow. There would not be noticeable damage, and you can
still easily remove it.
The cracks around the edge of coins is the most likely place for salts to be deposited, especially deep in the cracks. These are nearly impossible to clean, so the salt and clay/dirt stay deep. The only way to get to these areas, is long soaks, but you need to change the solution every 3 or 4 days.
Bruce