Hydroxyapatite is the molecule that your body transforms calcium into. It is the same material your bones are mostly made of (70% by weight), including your teeth. Nano-hydroxyapatite is it in a very small form. It can and will bond to your teeth in this form, remineralizing them. It can also, over time fill in small cracks in your teeth which is beyond fluoride. Also completely safe.

Fluoride on the other hand is toxic, it can remineralize at a lesser degree, and for the chemical process to work it has to be mixed with saliva. Which means longer exposure and also any fluoride in the water is utterly useless for "healthier teeth."

You can choose to have what your bones are made out of, or a nuerotoxin.