The Cussonia or cabbage tree is native to South Africa and used for bonsai due to the large fleshy roots. It likes a well drained neutral pH soil with lots of sun.
The pictures are helpful but do not show the white spots that you mention. What I do see is:
- veins of the leaf remain green but the area in between is yellow.
- it appears that new growth is affected more than old growth
This looks more like an iron or manganese deficiency, due to the pH of the soil being too alkaline, rather than a pest issue.
It is still possible that you have spider mites but we would need to see a closeup picture of the underside of the leaf where they live. Full details on what spider mites look and control like are found here but, in brief, they look like tiny grains of salt and are found on the undersides of the leaves where the flow of sap is strongest (near the veins). A really bad infestation will show webbing between the axils of the leaves. Fortunately a dish cloth soaked in soap and water rubbed on the underside of each leaf repeated three times at 5 to 7 day intervals will offer good or complete control.
Here is what you could try:
- An application(s) of a sulphate or "acid" fertilizer will help.
- Watering with pH neutral water helps in the long run.
- Top dressing with a more acidic soil based on peat moss or sphagnum moss helps too.