The Anti-Defamation League (ADL) references in its hate symbols database that the iron cross symbol goes back to the 19th century, included on medals for the German Empire. It was later adopted by Nazis for their medals, putting a swastika in the middle of the cross.
However, the ADL does note the ‘Iron Cross in isolation (i.e., without a superimposed swastika or without other accompanying hate symbols) cannot be determined to be a hate symbol,’ because bikers, skateboarders and extreme sports enthusiasts frequently use the symbol.