When growing emersed, one typically has to seat the plants in a bed of soil or some material to provide nutrients and to situate the plants in a stationary position. When there is mold, it indicates an excess in moisture. the substrate should be damp to the touch but not wet. The container should be sealed with cellophane to seal moisture. This is key as it prevents the plant in conversion to dry out.
Remove as much water from the substrate as possible. spray a 50/50 dosage of excel or 3% peroxide on the mold areas to clear it up and seal the plant container. Mist daily or every other day as necessary when either the plant or soil appears to be dry.
Fortunately, grass species plants are fairly resilient and they are tolerant to deep trims. Generally i trim as they advise with lawns which is to cut no greater than 1/3 each time. This helps the plant retain its nutrients while triggering root and runner to growth
"I am Groot", the faithful protector