Silver Shy Guy
From the Super Mario Wiki
Silver Shy Guys are a variant of Shy Guy in Paper Mario: Color Splash. Silver Shy Guys, like Gold Shy Guys, are invincible to all attacks except for Iron Jumps, Hurlhammers, and Things. Iron Jumps and Hurlhammers will cause their silver covering to fall off, revealing that they are really red Shy Guys. They act like normal Shy Guys once their silver covering comes off. Silver Shy Guys appear in Fort Cobalt.
When Silver Shy Guys are defeated, they drop more Coins and Hammer Scraps than most enemies.