Just disable recording changes. Recording changes is a feature to keep some "metadata" about edits concerning your spreadsheet. It allows you to track some data about which changes were applied to the sheet, so a third person may review and acccept/reject the changes. When recording is enabled, every cell that was changed since is highlighted by a thin red border, and a popup informs about recent changed.
You can control recording of changed using Menu "Edit
" -> "Changes
". There, you will find also a feature to disable recording of changes: disable "Record
". You will be asked for confirmation, but if you're not interested in the data recorded, you can safely confirm. "Data recorded" means the info in the popup box - date, time and content of edits regarding the specific cell, not the data in the cell itself.