Since it is mold, kill it first then take it off then sanitize it again.
Boiling water then vinegar then chlorine then rinse then a warm PBW soak and let it sit there for a day.
Rinse then sanitize. If there is still mold, you didn't do the PBW right. Do it again until it is gone.
Overkill? Yup, but then again it's mold in a stainless fermenter. If you do not get it clean you'll have strange problems in your beer until you get rid of it.