Yea i just started bjj and was wondering that too actually, I thought coach or something would be appropriate, but everyone calls the instructors by their first name/nickname. I guess everyones different because I had a boxing/muay thai coach that would get pissed if you called him by his first name. Someone would say "Andy, can you show me the clinch again." and he'd say " Listen to me, outside the gym you can call me andy or whatever the fuck you want, inside here you call me coach." I think that's understandable though, because there should be a level of respect. I don't know if i feel comfortable calling saying "Matt, can you show me what i'm doing wrong?" talking to matt serra.