Spritzing with. 'Shout' might get it out. Was it chalk made for marking, like clover, or was it kids colored chalk? If the kids kind it may be more difficult. I use chalk often, but I am careful to not get it wet or touch it with an iron. When finished quilting I use a soft brush or fabric eraser and remove as much as possible if any remains I might dampen a cloth with oxyclean stain remover or shout & rub the rest off. Hopefully if you try some spot remover the marks will come out