Animoji icon disappeared from message screen

Adding and removing sticker / Animoji / packs as favourites in iMessage

1. scroll to the very right end of the toolbar.
2. open the settings by tapping the … More button.
3. tap on “Edit”
4. tap the + icon next to your fave app.
5. tap the “Done” button.

Create and send an Animoji

• Open Messages and tap to start a new message. Or go to an existing conversation.
• Tap .
• Choose an Animoji. ...
• Look into your iPhone or iPad and place your face inside the frame.
• To start recording, tap . ...
• To preview your Animoji, tap in the top-left corner. ...
• Tap to send.

Use Animoji on your iPhone X and iPad Pro - Apple Support

https://support.apple.com/en-us/HT208190

Try restarting your iPhone.