Hi spas64,
Welcome to Apple Support Communities, and congratulations on your new iPhone. Rest assured, we're glad to help!
First, it's important to note that iOS features robust advanced automatic memory management features. It's recommended that you do not force close apps on your device, unless an app is totally unresponsive, or not working as expected. Force closing apps that are working as expected may have unintended consequences, as it may prevent them from exiting and saving their data properly. Force closing should only be used as a last resort when facing an issue.
Here's a guide with more information:
How to force an app to close on your iPhone, iPad, or iPod touch
Does your Galaxy Watch continue working as expected if you simply exit the app, and do not force close it? Is your new iPhone otherwise working as expected? What's the version of iOS installed? This can be found under Settings > General > About.
If we determine that your iPhone is working without issue, there's a possibility we may have to refer to the developer and manufacturer of your Watch and its app.
Have a great day!
