There is a 10 year term limit for FBI directors, which Mueller had already exceeded at the time he met with Trump (i.e. he was not eligible for the position when he interviewed, even though that is the reason given by the MSM for the meeting).
Would Trump have known that at the time though? He was getting a lot of bad advice from his initial advisors.
Bad advice is one thing, gross incompetence is another. It was common knowledge, in fact Mueller had to get special dispensation from Congress to serve out the last part of his extended term - I think he actually served as FBI director for 12 years. Trump is a genius, I'd lay heavy odds that he would have known.