There is no certification to my knowledge for martial arts. That’s the great thing about the arts its what you make it and choose to pass down to the next generation. However I would highly recommend getting some sort of national certification. This holds weight its fair to you and your clients health and safety. Teaches you a lot about exercise and the human body and different thoughts on fitness.
I was in the same boat as you in that I was teaching before becoming certified becoming certified was my best move and I would highly recommend putting in the time.
Hopes This Helps