If it is an idea (as opposed to a patentable solution), don't bother with an NDA. NDAs are notoriously difficult to enforce and only sends the signal that you are (potentially) a naive founder. There are certain situations where an NDA might be useful:
- When you have to discuss a patent-pending solution (typically where you have to walk through the entire process of building the solution or algorithm).
- When the expert may have visible conflict of interest (e.g., investor in a competing product).
- You need to have an NDA as part of another contract (e.g., maybe a client you are working with prevents you from sharing information with anyone else). This is obviously very rare.

Generally, if this is an app idea you're working on right now, it is better to be as open about it as possible and get clear and honest feedback from them. I have worked with over 300 individual founders and 60 early-stage companies in the last two years—and not one of them had any information worth NDA-ing. The benefits of brainstorming with an expert far outweighs the risks of the information going into the wrong hands.

Further, even if 0.5% of the people in the world have had similar ideas, that's 39m people who already have the same idea.