I would call him sensei, since he is a high ranking black belt in aikido and he did teach the sport for years. IF Steven Segal is teaching Anderson Silva aikido then there is nothing wrong with Segal being the sensei. Same thing, if Anderson Silva was in school for engineering and was in a multivarable calculus class he would call his instructor, professor. Just because Anderson is a great mma fighter does not mean now he does not have to respect other teachers just because their skill set may be inferior in an mma fight.