White on exposed roots

Hello, I just bought the six year old bonsai about two months ago and I’m starting to notice this white looking substance on the exposed roots of the tree. Could it be powdery mildew? Or something else? Please help

Do you have very hard water where you are? Lime buildup?

My house has a water purification system so I only use purified water. Should I use regular water from the faucet then?

That is impossible to answer without knowing what kind of purification system and what is the water quality before and after the system.