The link to the left is to a page about your coin, sadly, it is not a mint error. Coins from the US Mint of Philadelphia do not have a mint mark, that is how the Philadelphia mint is identified, by the lack of any mint marks.
Feel free to have more of your coins posted here, I can try to have a look at them these couple of days and send you details, with sources, for your coins estimated values.
The second link provides estimated values for your coin, I would say perhaps $1-2 due to its unique 'red' colour.
Also, in general, coins aren't to be cleaned, as it would destroy its value, although there is an exception to this rule with Ancient coins on a case by case basis, such as encrusted Roman coins, it be best not to clean any of them at all while you look through the collection.