Certified GKH Gardening Expert
It sounds like the work of a pest. I would treat the plant with neem oil. It will take care of any pest issues and is a fungicide as well.
This sounds kind of mysterious. Do you have access to any university botanical departments, or state extension services? You could take in some leaf samples and see what they say. Schefflera are generally tough customers;one thing is to make sure the soil is not too wet, because if the plant is strong and healthy it can fight off most invaders. The soil should be almost dry all the way to the bottom of the pot before you water again. Use a moisture meter, or a wooden skewer that you can push down into the soil - the meter should read almost dry, the skewer should feel dry and have no soil sticking to it.