Is it okay to address "Prof. X" as "Mr. X"?
This will depend on the norm within your country to some extent, but I'm inclined to say that at least in the US, calling a professor 'Mr.' is insulting, provided you are in an academic context. 

My neighbors, many friends, and the cashier at the cafe know me as Mr. X. But on campus, I'm Dr. X...and you know I'm a professor. Using 'Mr.' seems strangely intentional in a way to diminish ones accomplishments.