It depends on your state. As an RN, you are probably familiar with the RD credential, right? Some states it is illegal to practice without the RD, which is required for that state's licensure. Other states it is legal for everyone to practice, but you just can't call yourself licensed. That's why there are all these nutrition coaching certifications out there to give oneself a title of nutrition expert without going through formal nutrition training since the public doesn't know the difference.
I am a Registered Nurse with a great deal of knowledge about nutrition, but I still refer my clients to a RD. As "Joe Fitness" stated, most clients do not know the difference, but most states require the RD.
If it was up to me to seek nutritional advise, I would go to someone who is RD.
I would get certified. I just passed my exam today. I recommend looking at Precision Nutrition Level 1 certification. This certification would allow you to offer nutritional coaching. This is the best course I have taken in over 18 years.Course reviews all the science behind eating plus gives you all the forms to use for coaching someone. Good luck!