Hi Cristiano,
From your pictures, it looks like you are using the 'inverted polygons' option for the symbology instead of the 'rule-based- symbology (from the drop down options). This means you are currently applying the symbology to all the polygons that are NOT the same as the feature ID of the atlas feature page. By selecting the rule-based option and using the same equation, only the polygon that DOES match the feature ID will display the symbology.
Please let me know if you have any more problems.
All the best,
Josie and the GRID3 Training Team