I have been nursing a Jade plant cutting that dropped off an overwatered and rotting larger plant. It has been thriving in my windowsill for 6 months or so, and has grown a lot already.
In the last week or so, I have noticed a strange white speckling on the upper surface of almost all of its leaves. Upon close inspection, it does not look like insects; it looks sort of like a detergent residue, and if I scrape my nail against the surface of the leaf, a lot of it will come off, albeit with effort.
Do you know whether this is something I need to treat?
I wouldn’t assume the spots are a problem. As the following link to North Dakota State University Extension (now archived) mentions, it might be salt crystals that you are seeing:
“Those dots are salt crystals and can be wiped off with a damp cloth or just ignored because they are not causing any harm to the plant. All water (except distilled) contains some salt. When fertilizer is added to the root system, the plant takes up the nutrient salts with the water. As the water moves through the leaf pores during transpiration, the salts often are left behind on the surface.”
However, if you were to use a hand lens (not just the naked eye) and discover insects, there are resources with information on identifying and treating insect problems on indoor plants.
1. Pest and Diseases of Succulent Plants
2. Washington State University’s PestSense site lists several common houseplant pests, with information about treatment.
Always test any spray on one leaf before spraying the entire plant. Wait a few days after the test spray. Some plants are more sensitive to various soaps or oils.
3. The Northwest Center for Alternatives to Pesticides also has a guide to Growing Houseplants Without Using Pesticides.