I have a couple of Bonsai trees (Zelkova Schneidria)
which are starting to show a white powder-like substance
around the lower part of the trunk. I am not sure if this is
an indication that the tree is unhealthy , or merely a harmless
lichen. It is possible to scrape the substance off, but generally
it re-appears. Any advice? The tree is indoors, in a mostly
unheated room of the house where the termperature is around
2-8 C.

Jim