Q: This is on the trunk of our Japanese maple. Do you know what this could be?
A: It looks like there has been considerable damage to the trunk some time previously. I can?t tell whether it came from the trunk or from the roots. But in either case, wood rotting fungi is doing its normal job of decomposing the dead wood it found in the central trunk. This fungus did not attack the healthy tree parts.
Wood rot fungi appear on the surface of wood in several forms: mushrooms, thick shelves, or the white pasty stuff you are seeing.
There is nothing you can do to prevent the wood decomposition. The good news is that I have seen trees with extensive damage like this live for many years, as long as the outer layer of bark retained some semblance of health.
If the top of the tree seems normal, continue to give it good care, including watering in the heat and mulch over the root zone. No fertilizer is needed.