I have some kind of aphid. It looks likes small greyish blobs, very uniform all along the stems of the tree. Also, the lower part has some kind of soot on the leaves.
Certified GKH Gardening Expert
This is most likely aphids or mealy bugs. The black residue is sooty mold, which is caused by the pests. I would treat the plant with neem oil. It is effective against both of these bugs, as well as the sooty mold. Here is more information that may help you: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm, https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm, https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm, https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm