Through my experiences i have seen it mostly on hot water heaters. Dissimilar metals. that over time cause a green whitish build up around two fittings of dissimilar metals that have been threaded ,or sweated together. When it is touched it breaks up like powder. I have asked about this and the answer i get is use brass nipples instead of die electric nipples that some HWH's come with for connecting the hot and cold water lines.
My other question is: Is this build up of the funky green whitish stuff bad for the connection that can cause leaks and/or the pipe metal to break down etc.?