Sorry, the text of my post was erased when I added the photo. Can anyone identify this white sticky stuff on the leaves of my damson plum tree?

I would guess the sticky stuff is honeydew, aphid excrement. And there are a few exoskeletons and aphid bodies adhering to it to support that theory. The white speckling looks like it might just be pollen, which has also been trapped by the sticky substance. The honeydew is not a problem in and of itself but an indication of a past or current insect problem. There is also the possibility the honeydew could grow sooty mold but IME, that happens on evergreen foliage far more often than on deciduous.

Spraying the tree down with a strong stream of water can help.

Thank you so much, gardengal!

Whitefly infestation is another possibility, compare to these pics...

https://www.google.com/search?q=white+fly+damage&sxsrf=ALeKk00RoHVvBB-TroVtoeXPcjI0VURW9Q:1621890984374&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjPnvPfnuPwAhV4ElkFHdCMA0cQ_AUoAXoECAEQAw&biw=1280&bih=606