>Cancer actually means Krebs in german
This is true.
>which also means crab.
This is not. The word you're looking for is 'Krabbe'. Both in English and German cancer is a genus of crab but it doesn't mean it's the same thing.
And just for the record, I have heard some people who use 'Krebs' when they mean 'Krabbe' but I also hear people who say 'robot' when they mean 'android' on daily basis, doesn't make either of them right.