The Yeerk Peace Movement was a group of Controllers who sought a way of peaceful cooperation between the Yeerks and their hosts rather than slavery.
The peace movement was thought to have begun with Aftran 942, the Yeerk Cassie persuaded to leave her host, a girl named Karen, and live without a host. Later, Yeerks with hosts became a part of the movement, as evidenced in The Sickness, the book in which the movement's existence is first revealed. Yeerks involved in the peace movement are considered traitors by their fellow Yeerks and are subject to execution if they are discovered.
It isn't known what happened to the peace movement after the war, the final mention is that the Animorphs lost contact with Mr. Tidwell and the other members during the final phase of the war.