If a green border appears unexpectedly around a cell in a Google Sheets spreadsheet, then the worksheet is likely open elsewhere. If you've shared the spreadsheet, someone with whom you have shared it may have it open or you may have it open on another system or even in another tab within your browser. In that case the cell that is currently selected in the other open instance of the worksheet will be highlighted by a green border. At the top of the worksheet you will also see the number of other instances of the open worksheet. If it is open in just one other place you will see "1 other viewer" at the top right of the worksheet next to a green square.
If you move the cursor over the cell with the green border, you will see the name of the other user displayed, or your own logged in name, if you have the worksheet open multiple times. You can also put the cursor over the green squares at the top right area above the spreadsheet to see those names. If the worksheet was open more than twice, e.g., if it was open 3 times, you would see additional green boxes corresponding to the number of other open instances of it.