In iOS 11, Apple added an App Drawer to allow you to quickly access your message apps. The App Drawer is the row of icons that appear below your text box when composing a message.
If you don't use message apps often, you may want to hide the App Drawer.
Here's how to hide the App Drawer in Messages:
1 - Open a Message and Select the App Store icon next to the text box.
2 - The App Drawer is hidden. Select the App Store icon again to make it re-appear.