I have had some mold growing on the outside of the fermenter. Just wipe it down with a rag soaked in starsan and dry thoroughly. I also put a clean dry rag into the ferm chamber. Every time I open the chamber to check on progress I touch the rag. If it feels damp I'll pop it into the dryer for a few minutes. FYI, I once made the mistake of spraying the inside of my ferm chamber with starsan and LEAVING IT WET. Big mistake. Looked like mold central. The beer was fine, though.