BiMap<ElkGraphElement, Object> graphMap...;
ELKNode aNode...;
Object editPart = graphMap.get(aNode);
if (editPart instanceof AbstractDiagramNodeEditPart) {
    EObject siriusDiagramElement = ((AbstractDiagramNodeEditPart) editPart).resolveSemanticElement();
    if (siriusDiagramElement instanceof DDiagramElement) {
        EObject anObjectOfYourDomain = ((DDiagramElement) siriusDiagramElement).getTarget();
        String stringRepresentingTheSemanticClassName  = anObjectOfYourDomain.getClass().getSimpleName()
        ...
    }
}

ArrayList<ElkNode> nodes = new ArrayList<>(layoutMapping.getLayoutGraph().getChildren());
		for (Entry<ElkGraphElement, Object> entry : layoutMapping.getGraphMap().entrySet()) {
		    Object editPart = entry.getValue();
		    System.out.println(entry.getKey());
		    System.out.println(entry.getValue().getClass());
		    if (editPart instanceof DNodeListEditPart) {
		        EObject siriusDiagramElement = ((DNodeListEditPart) editPart).resolveTargetSemanticElement();
		        if (siriusDiagramElement instanceof DDiagramElement) {
		        	EObject ecoreObj = ((DDiagramElement) siriusDiagramElement).getTarget();
		        	System.out.println("Got here");
		        	System.out.println(ecoreObj.getClass().getSimpleName());
		        }
		        System.out.println(siriusDiagramElement.getClass().getSimpleName());
		    }
		}