Especially in cases where people try to bring attention to genocide in Palestine?

Why especially? Sadly also in cases like that, yes. German people are perfectly aware of their government stance on Palestine. If they want to see foreign, pro-Palestinian artists they have to elect different government. Blocking foreign artists does not endanger the right of Germans to vote in free elections. I mean, some people will say that it does but that's extreme view I disagree with.

And what is the difference between citizen speech and foreigner speech?

I already explained. Foreigners don't have a fundamental right to enter any country they want. You have visas and many rules around immigration. The government can decide not to let someone in if they think their presence is not beneficial. That's based on current policy, not rights. Policies are set by current government. Rights are guaranteed by the constitution. In a democracy people elect a government and it sets policy within the constraints of the constitution. Changing constitution is more difficult. In my opinion this is system working as intended. If majority of people in the country doesn't wont pro-Palestinian artists to perform in their country they can elect government that will ban them. As you pointed out, it will have little effect in practice. Banning citizens from talking about something is way more dangerous because it can have real impact. That's why it's protected by the constitution.