I don't know what you mean by looking like grass. Flattened out and not round maybe? That could just be pooping, but if it happens every night it could be a bad sign for health. Both mags and BTAs can do that. I wold look for parameter changes that coincide with your lighting. Temp, ph, salinity, oxygenation are all possibilities. If the water outside the anemone is different than the water inside it needs to expell the water and take new in.