I can't find any pictures that resemble these things on the web.
Following a closer look with a stereoscope, there are outbreaks of these
growths in at *least* a dozen places on this board. Here's two more
sharper pictures with the problem areas outlined:

https://www.flickr.com/photos/128859...in/dateposted-
public/

https://www.flickr.com/photos/128859...in/dateposted-
public/

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.

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