will eat - literally, “will have pasture.” The consuming progress of mortification is the image. They pretend to give rich spiritual pasture to their disciples: the only pasture is that of a spiritual cancer feeding on their vitals.
canker - a “cancer” or “gangrene.”
Hymenaeus - (See on 1Ti 1:20). After his excommunication he seems to have been readmitted into the Church and again to have troubled it.