Throw away the moss - you're probably overwatering like mad trying to keep it alive and drowning the scheff. in the bargain - and they don't like being watered until they really need it, which is when a good part of the soil is dry in the pot.

Repot it, probably root rot from wet soil, Schefflera doesn't really belong with moss IMO, after you pull it out cut off any black slimy roots, repot in fast draining soil.

wish you luck with your schefflera. i have some common schefflera houseplants acquired as gifts, but never thought they could be trained for bonzai due to problems with their branching habit (usually no more than one branch forms when trimmed). have you got any suggestions to encourage more branching?