Why is Eclipse highlighting my code in red and how do I turn it off?

This is activated because of code coverage. If you want to remove it then follow these steps.

1. Go to "Windows -> Show View -> Coverage" Select it. Click on "Open".
2. Open the "Coverage" view.
3. There are "X" and "XX" signs at upper right side. The "X" sign for remove single active session and "XX" for to remove all active sessions.
4. Click on one of them as per your requirement to remove the coverage session.

Refer the screenshot for more details.

Right click on project folder and then select on coverage configuration from configuration and deselect the source folder from configurations. Hope this will help you.