Sand bed and rock that is immature is quite likely to add bacteria blooms as well as spores for all kinds of things. Some of the bacteria is motile and swims too. I've had tanks get so cloudy from algae crashes and spores that you can't even see into them and then they'll be clear a few hours. I try not to solve things and let them work through cause these are all kinds of things that are growing in there. I also definitely don't filter or skim an immature tank.