A security guard at a Manhattan shelter was shot Monday afternoon near Madison Square Park — and police believe the assailant opened fire over an ongoing clash, law enforcement sources said. The victim, 49, was blasted in the torso just after 12:30 p.m. on Madison Avenue near East 28th Street in NoMad – steps from The Prince George, a shelter run by the organization Breaking Ground, where he works, cops and sources said. He was taken to Bellevue Hospital, where he was listed in stable condition.