By "Nazi Cross" I assume you're talking about the Iron Cross (please correct me if I'm wrong). The Iron Cross was used by the German military in the days of the Kingdom of Prussia and the German Empire, long before the Nazis. Of course, the fascist government in Germany attached their own symbols, namely the swastika, to official decorations like the Iron Cross. However, in 1957, the West German government reissued all military decorations awarded during the Second World War, replacing the Iron Crosses containing swastikas with medals containing oak leaves.

While the Iron Cross medal is no longer issued by the German government, the shape of the Iron Cross is still used in Germany today as a symbol of the country's armed forces. It has also become associated with biker culture, particularly in the United States. The use of the Iron Cross by motorcycle clubs can differ based on intent. While it is used as a white supremacist or neo-Nazi symbol by some, it also came to be used when American veterans brought Iron Crosses home as war loot, appropriating the symbol as one of rebelliousness and freedom.

So, I would ask your dad where he got the cross and why, and hopefully he can give you a clearer explanation.