It sat in my basement in storage for about a year, and must not have been cleaned well firstI soaked it in HOT water for about an hour and then scrubbed it with a Brillo pad, to no avail.Soak in bleach and water and scrub again with BrilloThe stain may remain, but the mold will be deadYou can cover the pan with
aluminium foil so as not to see the stain.Other related question