Alert to get below layer name under current later shape

I don't know if this is what you are looking for or not... The code is more of a proof of concept than anything:

var doc = app.activeDocument;
var oLayer = doc.activeLayer;

selectNextVisibleLayer(false);

function selectNextVisibleLayer(makeVisible) {
	var c2t = function (s) {
		return app.charIDToTypeID(s);
	};

	var s2t = function (s) {
		return app.stringIDToTypeID(s);
	};

	var descriptor = new ActionDescriptor();
	var list = new ActionList();
	var reference = new ActionReference();

	reference.putEnumerated(s2t("layer"), s2t("ordinal"), s2t("backwardEnum"));
	descriptor.putReference(c2t("null"), reference);
	descriptor.putBoolean(s2t("makeVisible"), makeVisible);
	list.putInteger(3);
	descriptor.putList(s2t("layerID"), list);
	executeAction(s2t("select"), descriptor, DialogModes.NO);
}

var nLayer = doc.activeLayer.name;
doc.activeLayer = oLayer;
alert(nLayer);

As JJMack noted: "what happens if the lowest layer is selected?"... In the case of this script, the top layer is returned!