Make sure you have good drainage in your growing container, perhaps even add some vermiculite to the soil mix. I use a *mister* to water more frequently without too much water. Its a pump one. You can find them at gardening centers, Home Depot, Lowes, WalMart. This helped me keep mold to a minimum.
What I was thinking however, was that the wheatgrass is stimulating this detox reaction and that you may need to use less and work up to a higher amount.