I have seen plenty of cards with a mark almost exactly like that one. I am not sure if soaking will get it off. That being said, if you can get the mark off and if there is an indention left, it will get the same MK qualifier. On the ones I have seen that were erased, similiar to yours, the indention was still there. I have a feeling a mars plastic eraser would take that mark off but not the indention left behind. No promises, I am just quoting my experience.
best regards