Fuga: Melodies of Steel 2

Silver and gold enemies are the flying tiny things that fly away from battle after a few turns as well as the spider enemies that upgrade their levels each turn. They're not particularly strong, however I'd recommend switching all 3 kids to Machine Gunners as then youll have a 50% chance to hit the flying ones. You'll be rewarded with random bits of sellable treasure for successfully defeating one or more of them in an encounter.

As noted in the comments, silver enemies are worth a lot of EXP, while gold ones give you loot upon defeating them (but no EXP).