What to do on the white line showing on my nose? (Photo)

When the tip cartilages are strong, and the skin is thin enough, you can see the outline of the cartilages, and the little dent between them. Working on the tip can help. The woman in the short video that I posted with this answer had her strong tip cartilages narrowed and made smaller in her operation.
You should understand that those changes require advanced techniques, techniques that most plastic surgeons cannot handle.  Be sure to read the section in the "Learn more" link on how to stay out of trouble while searching for a rhinoplasty surgeon. Rhinoplasty is an exquisitely difficult operation to get right, and you should only have surgery if you are able to make yourself very confident in your surgeon's skills. Read that page very carefully. 
Your nose is also a good example of why computer imaging is mandatory in rhinoplasty. You need to know exactly what the surgeon is planning to accomplish -- what features he thinks he can change, and by how much he thinks he can change them. When you see his goals, you'll know whether he has an eye for an attractive nose, and whether he shares your opinion of what constitutes an attractive nose. You'll also know whether the changes he proposes are enough to be meaningful to you, and whether he understands your wishes enough to address all of your priorities. But remember, you're not hiring him for his skills with the computer. The doctor must then show you his before and after photos to prove that he can actually accomplish what he draws on the computer.
In your case in particular, you need to find a surgeon who understands all of the issues I mentioned, and how they fit together, and who can actually accomplish them in surgery. 
I usually make computer morphs when I'm answering questions like this, to show the changes that are possible for a nose with a rhinoplasty, but of course you didn't post any photos. At the *very* bottom of the "Learn more" page is a link to instructions on taking and sending photos that are comprehensive and useful for an online consultation like this, in case you'd like a more complete evaluation.

Every nose is different and unique to that individual. If the appearance of your nose concerns you enough to do something about it, it can be improved with a natural artistically performed rhinoplasty. What would be the best option and exact procedure would depend on an in person examination (not just an online consultation) and discussion about desired outcome. Excellent results can be achieved with either a closed or open technique.
 
Keep in mind that following advice from a surgeon on this or any other website who proposes to tell you exactly what to do and how technically to do it based on one photo without examining you, physically feeling your tissue, assessing your desired outcome, taking a full medical history and discussing the pros and cons of the operative procedure would not be in your best interest. Natural appearing results need to be individualized and what is appropriate for one patient is not necessarily the best for someone else. I would suggest that you find a surgeon with extensive rhinoplasty experience certified by the American Board of Plastic Surgery and one who is ideally a member of The American Society for Aesthetic Plastic Surgery (ASAPS) that you trust and are comfortable with. You should discuss your concerns with that surgeon in person. 

Robert Singer, M.D., FACS
 
La Jolla, California

If you are considering changing the shape of your nose with a rhinoplasty that may also help with the discoloration.  A prominent cartilage can impact the way the skin looks via shadowing and/or blood flow.  I would have an in person consult with a board certified plastic surgeon to get a closer look and discussion about what bothers you most.All the best,Dr. Remus Repta