Line Trails

Hi. I am trying to make a line trail appear behind a symbol which is traveling along a motion guide. Originally I tried to just put in a bunch of keyframes and erase backwards, but that took up too much space. So, then I figured I could do it with a mask but I am not sure how. I want the mask to reveal the path of the motion guide as it goes. I am not sure how to do the mask for that. Thanks for the help.