We assume you mean yellow nutsedge, a tough grassy weed: http://extension.umd.edu/hgic/yellow-nutsedge It likes wet areas. That, combined with the fact that you have mold (growing on the soil?), suggests that the lawn is very damp. Try to improve drainage and air circulation (prune up some trees?)
Never let the yellow nutsedge form seeds/flowers or it will spread even more. Keep it mowed.
There are a couple of products that can be used for control it, but will probably need to be used more than once. Follow label directions carefully: Sedgehammer, and Dismiss.