When I was keeping l.caeruleus, they tended to ignore all vegetation and vegetables. In the wild they mostly feed on insects and small crustaceans, but will sometimes eat algae when bored/hungry. My suggestion would be to try it and see, they might, but probably won't. Sorry I can't give you a definate answer, but it's all a crapshoot with mbuna.
Holy Cow you only had the post on for a tad over an hour. Give us a chance to get on You may always see my name on here but I leave my screen open and rarely close it. Doesn't mean I'm always on browsing. There's not a ton of traffic here like in the tropical discussion section either.
But anyway. My labs I had picked at my moss a little bit. Not a whole lot. And as Dawn said it's really a 50/50 chance. Then there's the chance that they don't eat it now, but down the road they decide they like it for some odd unknowing reason.