Hi! Today I noticed that my two schefflera plants both have strange spots that look like dirt on their new growth (as shown in attached photo). They aren't scale or mites-- they won't scratch off (though one plant did have a few spider mites on it elsewhere. Sigh). Nearby schefflera cuttings in water don't seem to be affected by these spots. Any idea what they are or what I ought to be doing?

More information: These two plants currently live in a south-facing window in Seattle. Two months ago, they moved from a shaded south facing window in Sacramento. They didn't seem to have any particular difficulty adjusting to their new location, and have been putting out quite a lot of new growth. I fertilize them lightly once every two weeks, and I pruned them about two weeks ago. These plants have been in good health for quite a while. I'm stumped. Thanks in advance for any insight!