The following link is to an article from Michigan State on Mold Control and cleaning in Walk-in coolers. (PDF)

Tips to help slow or minimize mold growth in walk-in refrigeration units:
• Limit all paper products, agars, medias in the unit — these are excellent sources
of food for mold
• Remove/dry any excess water from the unit to assist with moisture control
• For long term shut downs (greater than one day), thoroughly clean the unit with
dish detergent and keep the doors of the unit open to allow for complete drying
of the unit. Do not close the doors until the unit is ready to be re-entered into
service