Sound like powdery mildew on your plants. It happens a lot, especially if your plants are too close together and you water in the late afternoon or evening -- humidity is the main cause. Once your plants have it you have a chance of managing it, but it's nearly impossible to get rid of it.
Find the leaves that the mildew has completely overtaken (check the underside of the leaves, too) and cut those out. Put them with your yard waste to be recycled or throw them away -- but don't compost them. Also clean any tools you use before using them on other squash, cucumbers, etc... -- powdery mildew spreads easily.
The solution to use for treatment when your plants have powdery mildew also can be used as a preventative treatment. In a 32 oz. hand spray bottle mix 6 oz. of milk, 2 teaspons of baking soda and a drop of dish detergent (not dishwashing detergent). Fill with water and mix well. Spray on tops and undersides of leaves. Repeat weekly. Store in the refrigerator.