To make complete…. First I dont’t have to use the standard data-layer, it takes me while to find out that this is the answer to my question about layers:
GridLayer = new woosmap.map.Data();
GridLayer.addGeoJson(geojson);
GridLayer.setMap(map);
Than I can set the map to null for that data-layer.
GridLayer.setMap(null);
I thought that there was only one datalayer, it was not clear for me that I can create different ones.