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Try a tiny bit of alcohol on a q-tip. Barely moisten the end, and rub only the area with the spot. Wipe down immediately with a wet cloth/napkin.
Also, and I hesitate to recommend this, especially if you're not used to using it, but if the alcohol doesn't work, put a tiny, TINY amount of nail polish remover on a q-tip and rub the spot only, not the surrounding plastic.
I can't stress enough that you must use it sparingly. Nail polish remover contains acetone which will eat the plastic in nothing flat, but a trace amount on a q-tip. and an immediate wipe down with soapy water will do the trick. Use this as an absolute LAST resort.