Grammarly is awesome - English was never my strong subject at school so I'll take all the help I can get.
However... it seriously messes with the editor we use in PIMMS so I recommend that you disable it for PIMMS. Luckily, it's easy to do.
You'll notice on your browser's toolbar that there is a little green button.
Click on the green button and a popup window will open
Slide the green switch by the word "enabled" to the off position and Grammarly will no longer be active when you are in PIMMS but will still work wonders in email etc.