Can you treat eczema with wheatgrass?

No: I am not aware of any reference to the use of wheatgrass. Essential fatty acids flaxseed oil can be very helpful.

Eczema: I do not feel that wheatgrass will have any benefit for eczema. Best thing to do is to keep the skin lubricated with a lotion such as cetaphil or cerave, avoid deordorant soaps and switch to dove, and try over the counter hydrocortisone. Next step your doctor or dermatologist.