When you're cycling a tank with no fishes, you need to "feed" the fishes that aren't there to keep the bacteria fed, so it is known as "ghost-feeding".
Did you rinse the substrate really well, until the rinse water water ran clear? I suspect this is the issue since the tank is so young. It will settle out eventually...give it a few days.
Was your LR cured or "raw"? It might also be a bacterial bloom, but I'd think suspended fine particles first (add some floss in the drain area if you can do so).
Either way the tank should clear up soon.