Thank you both. There are ants all over the tree as well, so I guess they're going for the honeydew secretions, though the leaves don't seem sticky at all. I was afraid these were mealybug cocoons because I've read that ants like to move mealybugs from plant to plant and that could be a disaster. I guess cottony scale is not so bad.
Cottony scale honeydew, besides feeding ants, can also result in black Sooty Mold's covering leaves and stems. I don't know how damaging that is to the plants, but it is very unsightly. I have this problem on my Camelia and hardy kiwifruit vines.