What happens when a dwarf umbrella tree gets too wet

Q: I purchased a Schefflera arboricola dwarf tree two weeks ago from a nursery. The soil was wet at purchase, two weeks have passed, and the soil is still moist! I don’t want to overwater, so I’ve left it alone. Worse, leaves have started to turn brown and fall off (but they are not dry or crispy). I have it indoors in a sunny (indirect light) and warm room. What can I do to help my little tree? I enclosed some pictures so you could see the leaf situation. It’s a lovely tree an