There isn't enough information to nail down the answer, but it is likely that it is a bacteria bloom. Fungus is normally associated with egg failure, but the cloudiness is typical of high bacteria numbers. In either case, one factor or more is not optimized. Under filtration and inadequite maintenance combined with a sudden increase in organics can cause bacteria blooms. Substrate without UGF plates will harbor anaerobic bacteria that can quickly bloom with the increase of organics. Many bacteria can thrive in both anaerobic and aerobic environments. When they become aerobic, they consume large amounts of oxygen in the water and can cause suffocation in both the parents and the eggs (the eggs are much more prone).