[Arid_gardener] mexican fence post fungus (?)

[rco at swiarchitects.com]

I have a mexican fencepost with three beautiful columns, about 4 ft high.  One column has scale (?) or fungus (?) growing for sometime now (maybe two years)  slowly up the length.  At first I wiped it off with my finger, but that seemed to burn the plant and left a dry patch. Soapy water spray didn't have any effect, good or bad. Can I simply brush it off gently with an old soft toothbrush and rinse with water?  I don't want it to spread to the other columns, which are doing just fine, but this particular one is becoming completely covered in tiny white fuzzy stuff.  Please help. It is one of my favorites.  Thank you.