Congratulations, Opendock. You have a valuable coin. If your coin is in superb numismatic (coin collector) condition, like the coin in our photo from Westfalische Auktions in Bremen, Germany, it is worth hundreds of US dollars. In poor condition, the value goes down substantially.
NEVER CLEAN A COIN. CLEANING RUINS VALUE.
The old German State of Nassau minted these 5 kreuzer coins only in 1808 and 1809. The 1809 date is common. You have the rare 1808 date. Here is what the catalogs say about these coins:
5 KREUZER DATED 1808
worn: $200 US dollars approximate catalog value
average circulated: $400
well preserved (our picture): $1000
5 KREUZER DATED 1809
worn: $20 US dollars approximate catalog value
average circulated: $35
well preserved: $100
These are catalog values. Use our Important Terminology page to understand what this means. It is a mistake to think that catalog values are actual values.
The Westfalische coin is currently for sale at this link for 875 euros (about $1200 US dollars) which shows the importance of being in good condition.
Remember that the $1200 figure is a retail price. If Westfalische or any other reputable coin dealer were to buy your coin or one like it, the amount offered would be substantially less. This is standard retail markup, and is needed to keep coin dealers in business.