Cleaning coins and a Counterfeit detection pen mark on a 1928 bill?

I count money for an armoured truck service and I have a small collection of old bills and coins that I have had the privelage of finding and buying from my employer and my most exciting one is a 1928 that says "redeemable in gold" on it…very cool. It is in great condition other than that annoying mark from a counterfeit detection pen. Is there anyway to get it off without damaging the bill?
Also does anyone know what is the best way to clean silver (older than 1968) half dollars?
Thanks

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