As far as unwanted markings go that looks professional and not too bad so I would leave it.
If it is really bothering you take an exacto knife and remove one small portion to see if it is paint or a sticker and then try different solvents to see what dissolved it without risking the case.
Be aware that is that marking has been on the unit a long time and the case has discolored at all then removing the marking will still show a shadow in the plastics because it blocked sunlight from discoloring the plastic underneath.
Thanks guys. I will experiment on a small section. I expect to retrobrite this Mac as it is because of needing to replace the faceplate too and the color doesn't exactly match. So I will get the case one uniform color one way or another.