I finally found why my Elm began dropping leaves, and found some info
for spider mites here, which I've done - I placed a clear bag around
the tree and top of the pot. I sprayed it with water first, and put
it outside on a windy day to blow all the loose leaves away.

Now I'm wondering - how long do I keep the bag on there? It's been
about 4 days.

--
Spectre