One of my iPhone's apps is using hidemyemail and I don't know why

This is a part of iCloud+. Probably you are unaware of the advantages of this feature.

Hide My Email offers several advantages:

1. Enhanced Privacy: Hide My Email allows you to create unique, randomized email addresses that can be used for different services or online platforms. This helps protect your actual email address and reduces the risk of it being exposed to spam or unwanted communications.
2. Spam Prevention: By using Hide My Email, you can avoid providing your real email address to websites or services that might sell or misuse your contact information. If you start receiving unwanted emails or spam to a specific Hide My Email address, you can easily disable or delete it without affecting your primary email account.
3. Control Over Communication: Hide My Email gives you greater control over who can contact you. If you no longer wish to receive emails from a particular sender, you can disable or delete the associated Hide My Email address, effectively cutting off their access to your inbox.
4. Reduced Data Collection: Many online services require an email address during signup, but they may not necessarily need your real email for their core functionality. By using Hide My Email, you can limit the personal information you share and reduce the amount of data collected by various platforms.
5. Organization and Filtering: With Hide My Email, you can create unique addresses for different purposes or platforms. This allows for better organization and easier filtering of incoming emails. For example, you can create a separate Hide My Email address for online shopping, social media accounts, or newsletter subscriptions.
6. Easy Management: Apple's Hide My Email feature is integrated into the Mail app on iOS, iPadOS, and macOS. This makes it convenient to create, manage, and delete Hide My Email addresses directly from your device settings without needing to rely on third-party services.

Having said that you can always choose not to use Hide My email and delete them whenever you desire.

Please refer to the pic below which is self-explanatory

To deactivate Hide My email, please see the pic below

If you have enabled iCloud+ then this feature is part of the package.

There is no option to turn the "Hide My Email" On/Off. You do have an option to either use it or not use it. The choice is yours.

I always use this feature and feel secure and protected from Spammers because I can always change the "Hide My email" ID whenever I want. My parent email ID remains the same, which is unknown to the world.

A great feature that I admire.