In your ExpressVPN app’s diagnostics file, you may see the following warning:
WARNING: this configuration may cache passwords in memory -- use the auth-nocache option to prevent this.
The ExpressVPN app caches the username and password of your OpenVPN configuration and reads them directly from your device’s memory so that there is no need for you to enter them every time you reconnect to the VPN.
Your device’s memory is safe from unauthorized access and is not obtainable without physical access to your device. Since this message does not indicate that your privacy and security are compromised, you can continue to use the ExpressVPN app normally.