Mould forms from the seeds that don't germinate. This happens whether you grow them inside or outside. Some trays are better than others, but they guess 7-10% rot and the mould forms. The mould is then picked up by the roots of the seeds that do germinate and are inside the grass as well where you can't wash it off. This is what gives it that nasty bitter taste.
If you can grow our grass outside, preferably in the ground, leave lots of space between seeds. Sunshine will actually kill the mould.
The other benefit is that when planted in the ground, it can mature from a sprout to a vegetable, so it doesn't have the sugary sweet taste, but instead is very smooth like green tea.
Personally I react to indoor grown wheatgrass juice, and was told I was toxic and having a healing crisis, but I don't believe that anymore as you can't detox within seconds.
Even when I have the green powders and they use indoor grown wheatgrass juice, I want to throw it up.
I stick with my outdoor wheatgrass and using other greens like Kale, celery, and parsley in my juice.