Certified GKH Gardening Expert
It is most likely due to mealybugs or spider mites. I would treat the plant with neem oil. It is effective against all of those pests and many others, but does not harm people and pets. Here is more information on it: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
As for watering, rubber tree plant needs to be kept moist but not wet, and allowed to dry somewhat between waterings. You can check this with your finger. It is also a good idea to spritz it with water. Note: If you water too much, the leaves will turn yellow and brown and fall off. During the dormant season, it may only need watered once or twice a month.