All the pictures I've seen on the net show whiskers as silver in colour
but these are dark, metallic plum. They typically spring from vias in a
kind of branching, fern-like formation and certainly reach out to touch
other traces on the board.

Cracks?
Anyone seen anything like this before? Ideas on how to proceed?

Choose one of these, try acetone on a Q-tip cotton ear bud to dissolve the
coating (try to not disturb the €œthing€� that lies below the coating--if,
in fact it is below the coating). Get a €œburnishing brush€� with
fiberglass bristles (used to clean oxidation from copper, etc.) and start
gently rubbing the site. I think youll quickly discover if its a
surface growth, or a crack or other phenomenon.

Good luck.