I could in fact be a bacteria bloom, in which case watch your fish for signs of distress like gasping and swimming near the surface, as a bacterial bloom can remove vast amounts of oxygen from the water.
It could also be dust from the phosgard, or a calcium precipitation event - but that would likely entail some other event like an overdose of Kalk or two-part. Increase flow in the aquarium and run a filter sock if you have one, if it doesn't start to ebb - likely bacterial. Increase skimming (wet skim) and do a water change...