Is the plant pot-bound? If a bonsai's pot is full of roots, the roots start pushing the base of the tree up. This might be causing the cracks. How long ago was it re-potted?
Also a couple of things: Certain soil "cracks" more than others when they are dry. You can see this in nature when clay dries, it cracks. Clay has virtually no organics, so make sure when you re-pot that you are using soil with enough organics to hold the moisture, but enough inorganics to allows for good drainage.
Another observation" Your moss seems to be drying, declining, or dying. Do the cracks go beneath the moss? It appears the moss around the feeder is in better shape either to do with it receiving better nutrients or simply more moisture. I hope this helps
Cheers!.
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