The white stuff is probably dead bleached algae carcasses, or sand, or dust, or DE or something else inert. Vacuum it up before they stir things up and make it look cloudy. I doubt it's toxic.
As for safe... depends on what "all levels are good" means. If the pH is between 7.2 and 7.8 and the FC level is below shock level for your CYA, then let 'em in.