I have several cacti growing in my yard and one (t. pachanoi, if it matters) has developed a white layer across the top of the plant. It is very fine and looks almost sandy, like perhaps a mould or fungus. It does brush off fairly easily though.
What is this white stuff? Is it dangerous to the plant or its neighbours? How can I treat it?
EDIT: Now with picture. I've brushed most of it off but you can see the white dots across the top. Before my attempt to clear it it was covering the entire surface