From SmashWiki, the Super Smash Bros. wiki
Amaranthine gas is a dangerous substance found in certain caves in The Subspace Emissary. When characters enter the purple mist, they quickly start taking damage at about 2% every half-second, depending on the difficulty. However, characters do not flinch while in the gas. Whenever amaranthine gas is present, there is always a switch nearby that when hit, will temporarily clear the gas; the length of time the gas is deactivated depends on the difficulty.