How do I get rid of the app attribute in my order details?

All products added to the shopping cart using the ' Add to Cart' button in that app and then purchased include the Shopify cart attribute '__Added with Frequently Bought Together button'.

This attribute can be used for app auditing and will appear in your Shopify order details page:

The double underscore prefix is a Shopify convention that tells your Shopify theme and other third-party apps that it should not be displayed to your customers.

Anyway, if you prefer to not having that auditing attribute you can disable easily from the app preferences page, just click on the "Tracking preferences" link, placed on the Statistics at bottom right.