Looks like mineral deposits left from evaporating water. That's common after summer where water contains levels of dissolved minerals. More common in hotter climates where trees are watered more often and evaporation is high. It can be minimised by flushing the soil with lots of water every few weeks. Much worse where trees sit in a saucer of water that allows more water to evaporate through the soil each day.
Once the minerals have precipitated it can be hard to get rid of the residue but some is likely to dissolve and disappear over cooler months. Not sure if that applies in Singapore though?