Moisture promotes mold. When you are soaking up the seeds before plantation, do not soak them up for more than eight hours. Some people choose to leave them overnight and even for days. This encourages mold growth. When the seeds themselves become prone to mold growth, it is hard to eliminate the mold.
Grapeseed extract is known to be very effective against mold growth. When you are watering your wheatgrass, add 20 drops of grapeseed extract to every 32 oz of the water. When the plants take in this mixture, they will become more resistant to mold formation. The extract will not harm the plant growth in any way.
Temperature control can also help you avoid mold. Do not grow your wheatgrass at above 80 degrees. High temperature increases mold growth. However, drastically lower one will result in low wheatgrass growth. Maintaining a temperature between 60 to 75 degrees is going to have perfect growth without any mold formation. If you need to lower the temperature when it is hot, place a fan over the wheat grass and use the air movement to drive down the mercury.
Get a seed variety that is mold resistant. You might not be able to get one that is completely mold resistant but there are some which are better at the job. Select the variety after consultation with friends and those with knowledge of herb growth.
The trays in which you harvest your seeds should be thoroughly cleaned. Use grapeseed extract, bleach, hydrogen peroxide to clean the trays after you harvest the seeds. Any contaminant on the trays is going to encourage mold formation.