those are droplets of cooking oil that have condensed on the metal grille. They come from the smoke that is made while frying food. The best way to remove it is to take down the grille and put it in a dishwashing machine. The dishwashing detergent is designed to dissolve cooking oil deposits.
If you don't want to throw it in your dishwasher, you can try soaking it in a tub full of soapy water for a while then scrubbing. That's what we do. We also have, in the past, soaked in vinegar. Soap works better for us.