That means that you're using the GMX website to access your email, rather than any proper email app. So-called 'webmail' has always been second best to using proper email software, especially on mobile devices.
I went on the app. I can't seem to log out of it. I have 2 accounts on gmx one is personal and one is for all else. I can only appear to get the one account now
You don't log in and out of most mobile apps. They're not designed that way. It's the security on your phone (such as its facial recognition feature and/or your PIN) that protects your account.
Using the app, rather than webmail, might seem a little awkward at first but I can assure you that you'll actually find it far easier once you get used to it. Further, you'll now get immediate notifications for incoming emails, which you wouldn't have had when using webmail.