We had an addition put on our cottage, on Lake Huron, maybe 8 years ago. New log siding was used. The siding was sealed and then stained. On the east and a bit on the south side of the cottage, ledges of growth-maybe a lichen(?) has been growing this year. Upon breaking a few pieces off, it looks like part of the wood/log is breaking off with it. On the south side of the cottage there small pieces growing (full sun). The east side has multiple larger pieces (morning sun and near a corner) but in the same corner the wall facing south, also newer wood, it's ok.. The old wood, from the 1930's, was stained or painted with some concoction of stain, oil? and creosol, I believe. The old log siding is uninfected. Please, what is this and how do we prevent and/or kill it?