We grow a few different cacti here and they often get MEALY BUG. Perhaps
your cacti has mealy bug, which are white and fluffy and hang out in the
crevasses and difficult to get at parts, in clusters or lines along the
stems. Try and wipe the stuff and if it is very soft and has coloured
guts inside (usually orange,yellow or red) then it may well be mealy bug.
Mealybug can be controlled with a spray of white oil, or white oil with
pyrethrum and/or methylated spirits. (Or nasty chemicals!)
Dept. of Natural Resources