The Vomit Grimace is a green-tinted version of the Stone Grimace added in the Wrath of the Lamb DLC. They can replace regular Stone Grimaces, and naturally appear in some rooms in Necropolis and Utero.
Their behavior is exactly the same as normal Stone Grimaces, but instead of firing a blood shot at Isaac, they fire an IPECAC shot instead. This shot's explosion can be used to kill other monsters in the room, break rocks, open Stone Chests, or open any Secret Room/Super Secret Room entrances.
When having cleared a room with Stone Grimace it is possible that one of them changes to a Vomit Grimace until leaving the room.