The main reason why Trump is afraid of Robert Mueller is because there is no telling what Mueller is going to do next. It was in the year 2014 when Mueller and his team formulated a goal which is to make sure that Trump wins the elections. They started using social media sites particularly Facebook in order to create division among the Americans.
Americans started questioning what they should actually get. As a result, Trump won the elections. Trump is not recognized all over the world as a good leader. In fact, most countries view him as the opposite. It will not be surprising if exploitation will happen in the future.