A white wine that was clear once but now is hazy or is hazier when it cold than when it is warm is suffering from protein haze. This is largely an aesthetic problem but one which winemakers try to eliminate. This article explains what home winemakers can do.
There is general agreement that white juice should be fermented at cool temperatures, taking several weeks to complete the process rather than the several days common with red must. Keeping white ferments cool is particularly applicable for the aromatic style. The cooler ferments can yield better varietal fruit flavor.
Yeast requires nitrogen for fermentation. If there is not enough, yeast cells are stressed and produce excess H2S—something that gives fermentations an off-odor. If you want to avoid problems, you must be prepared by adding nitrogen to fermentations.