Hello, the attached picture is of an indoor Texas Ebony that I've had for about a year. I estimate it's 15 years old. Since a few months Texas one branch is showing white spots and the twigs/leaves are slowly turning yellow and dropping. Sometimes I spot a small ant 'tending' whatever the white stuff is. Can someone help me identify this disease/pest and tell me how to treat it? Thank you.

Wooly aphid or mealy bug. and also scale insects, either way a serious infestation that will weaken this plant or even kill. You need an insecticide preferably systemic.