If it is white and doesn't move when you touch it then it is probably a scale type of insect. Lighter oils, plug up the breathing pores of the scale yet do not harm the plant leaf. If the pest is white and it feels wet or slimy, it is probably a spittlebug. Well, there's no other way to describe the protective cocoon of stuff they hide in. If it is white and barely moves or crawls away very slowly, it could be an aphid or perhaps a mealybug.
Mealybugs look like little chunks of cotton batten.
Both are easily controlled with soap or light horticultural oil sprays. Now if it is white and it feels dry or powdery looking, it is probably powdery mildew. Powdery mildew is a mildew fungus that loves to live on garden phlox, lilacs and roses. If the foliage of a plant tends to be damp in the morning because of its location, it is a candidate for this problem. Early watering allows them to dry out in the heat of the day while late eveing watering allows the fungus spores to find time to establish themselves during the dark, humid night.