Certified GKH Gardening Expert
It likely is a fungus (there are several that create brown spots) and should be treatable with fungicide.
The most likely fungus culprits flourish in damp soil, so for long term control, try not to water so much. Schefflera really detest wet roots. The soil needs to be fully aerated before watering again; that is, if you dig up a spoonful of soil from as far down the pot as you can reach, at least 6", it should feel cool and pleasant, and if you squeeze it, it should clump together not at all, or only slightly.