"So this works on whiteboards... If you leave something on too long, it will leave what you are seeing. What I do is take the marker and just go over it again and quickly wipe it off. It tends to "lift" the stained part and since the new mark isn't fully dry, it comes off. You could try it on a small section to see if it works.

FYI, this also works if someone uses a permanent marker on a whiteboard. Use the "Dry Erase" marker to lift the permanent marker and get it off the board."