We have been growing wheatgrass in our kitchen for several months now. For a long while, mold was a big problem. We tried many approaches to mold control - including spraying with 3% hydrogen peroxide - all to no avail, but recently we've found a solution that really works well.
The key is in keeping the top surface of the soil dry. We put our soil mix in one of the slotted black plastic trays, and place that tray in a second tray without holes. Watering the grass is done by lifting the top tray with the wheatgrass and soil in it and pouring water into the bottom tray. A "light" watering fills only the depressions in the lower tray. A heavier watering means enough water to cover the ridges to a depth of 1/16-1/8".
How light or heavy the tray feels is a good way to judge the need for water, keeping in mind that as the grass grows taller, it gets a bit heavier, so a tray of thirsty 8" grass feels a little heavier than a tray of thirsty 2" seedlings. Also, we avoid watering on the first day new seedlings are uncovered, to allow the surface of the soil to dry out. (If the soil is wet enough when the seeds are first planted and the tray is kept well-covered, there's enough moisture left when the new seedlings are first uncovered to let them go a day or so.)
So simple, and an added benefit is better control over watering. We've only had traces of mold on a few of the last ten trays. It's still possible to get more vigorous mold growth - by overwatering. Excess water soaks up through the soil to the surface, as well as emerging from the tips of the blades and dripping down. But as long as we keep the soil surface dry the "bottoms-up" watering technique works like a charm. Mold is no longer a problem!
Alan and Marie Ford