This is most likely a corrupted category record in the WordPress database.
First, back up your database. Then go to phpMyAdmin → your WordPress database → wp_terms and find the category by its existing slug. If the term is there but the name is blank or showing incorrectly, edit that row and put the correct category name back in.
Then check wp_term_taxonomy and make sure the same term_id has taxonomy set to category.After that, clear your WordPress/site cache and check Posts→ Categories again.
Do not delete and recreate the category yet, because existing posts may still be attached to that category. Repairing the existing database record is the safer approach.
