To make it quicker for user to access their stickers, gifs and apps, Apple changes the iMessage chat screen in iOS 11 by placing a row of app icon at the bottom.
The icons on the app row are colourful and the look does not blend well with the text message input row. By default the app row is displayed when you enter a chat window. Luckily you can toggle to hide it from appearing at the bottom.
To hide the app row, simply tap on the iMessage App Store icon next to the camera icon. The app icons will stay hidden until you tap on the App Store icon again.