Most likely what you’re finding is not mold, but is a deposit that results when someone misuses an acid cleaner on fixtures, stripping away the plating and exposing the metal below. Strong alkalines also could degrade metal plating.

This condition creates a green build-up, most often a metal sulfate if the fixture is copper or brass with a chrome plating. As the exterior plating wears off from use of sulfuric, phosphoric or hydrochloric acid, and the metal oxidizes, it will create a crystaline or lattice structure that can look “fuzzy.” This condition almost always is the result of misused cleaning chemicals, but there are a few other possibilities cleaners can look into. One is that continuous use of pumice has worn down the plating rather than acids. Another remote possibility is that there is a build-up of an overly strong quaternary disinfectant that was never rinsed or polished off and may form green crystals when caked on.

Since most options point to damaged metal plating, there is little one can do to rid fixtures of this green deposit. A short-term solution is to remove the discoloration by rinsing or neutralizing with a moderate alkaline cleaner and then water rinsing. Another longer fix uses a wax or Krylon spray coat to keep the green from reappearing for a while. But, ultimately, such fixtures will need replacement and proper cleaning with less abrasive cleaners once they are installed.

Tips offered by Jeffrey Wooldridge, Housekeeping Supervisor II at Colorado State University.