The red underline is called "spell checker", and it's an Android feature. It typically only works with the pre-installed keyboard decided by the OEM. Sadly, it won't work with most of the 3rd party keyboards, including Fleksy.
Follow the steps to disable it:
- Phone settings
- Language & Keyboard
- Find spell checker
- Turn it off
Normally, you won't see them again. If you do, we recommend contacting your device manufacturer.