Do NOT use acne cream! It continues working long after you think it's been removed and can transfer to other ponies causing a great deal of damage. Your best bet is sunfading. First remove any surface ink with a magic eraser or acetone. When the ink is fairly fresh, this will often remove most of the mark. Older stains (as this appears to be) will have seeped into the plastic and require sunfading. Unfortunately, the mark on your Masquerade is not an easy color to fade; so, it could take a very long time if it comes out at all, and you'll need to protect the rest of her from fading as well.