Use "Tags" menu "Replace" to search any selected cells or replace text in your custom tags with the white background. If you want to search all cells, you first click the upper left cell, then scroll to and shift-click the lower right cell.
Enter the text you want to find and click the "Find" button. The next matching text in a selected cell is highlighted. Clicking "Replace" will replace with the text from the "Replace With" text box and advance to the next matching text in a selected cell. Click "Replace All" to replace matching text in all remaining selected cells with no further prompting. If there's no more matching text, you'll be asked if you want to start again from the beginning.
You may easily create custom tags from filenames by copying filenames to your new custom tag column and using "Replace" with regular expressions.