The local nursery should have known that spider mites do not secrete sticky stuff, but mealybugs and scale insects do. Without a photo, I cannot be sure, but if the white stuff looks little bits of white fluffy cotton, then they are mealybugs.
Mix a solution of 5 parts water, 1 part alcohol and a squirt of liquid dish soap in a spray bottle. This spray will be effective, but only if you are really thorough so that you cover all leaf and stem surfaces of the infested plants until they are dripping wet. This is a messy job best done outdoors in the shade, if possible. If you miss a few of the critters, they will reproduce and come back again later, so be really thorough.